I don't really get the question so I hope this is what you wanted. If you include a full document (such that we copy paste and see the problem on our systems) things are much more easier.
Here, you can change the default setting within a scope but your block style had a node distance which was resetting every time it is issued. I've made it 2mm such that we can see the difference easier.
\documentclass[tikz]{standalone}
\usetikzlibrary{arrows,shapes.geometric,positioning}
\begin{document}
\begin{tikzpicture}[decision/.style={diamond, draw, text width=4.5em, text badly centered, node distance=3.5cm, inner sep=0pt},
block/.style ={rectangle, draw, text width=6em, text centered, rounded corners, minimum height=4em, minimum height=2em},
cloud/.style ={draw, ellipse, minimum height=2em},
line/.style ={draw,-latex'},
node distance = 1cm,
auto]
\node [block] (1st) {1st};
\node [block, right= of 1st] (2nd1) {2nd1};
\begin{scope}[node distance=2mm and 10mm]%Here we change it for everything inside this scope
\node [block, above= of 2nd1] (2nd2) {2nd2};
\node [block, below= of 2nd1] (2nd3) {2nd3};
\node [block, right= of 2nd1] (3rd1) {3rd1};
\node [block, above= of 3rd1] (3rd2) {3rd2};
\node [block, above= of 3rd2] (3rd3) {3rd3};
\end{scope}
\node [block, below= of 3rd1] (3rd4) {3rd4};
\node [block, below= of 3rd4] (3rd5) {3rd5};
\path [line] (1st) -- (2nd1);
\path [line] (2nd1) -- (2nd2);
\path [line] (2nd1) -- (2nd3);
\path [line] (2nd2) -- (3rd3);
\path [line] (2nd1) -- (3rd1);
\path [line] (1st) -- (2nd1);
\end{tikzpicture}
\end{document}
Let's start with an example:
Gantt charts a, b and c use next declaration:
\begin{ganttchart}[ x unit = 0.30cm,
y unit chart = 1.2cm,
y unit title = 1.2cm,
hgrid={ultra thick,draw},
time slot format = isodate-yearmonth,
compress calendar,
title/.append style={opacity=.5},
]{2013-05}{2013-12}
The important parameter is y unit title (in your example is not explicitly declared, then fixed to its default value: 1cm). This parameter fixes the height of every title bar.
To better understand what happens, I've also drawn some red lines with distance 1.2 cm (y unit title) between them.
\gantttitle[title/.append style={fill=blue!15}]{2013}{8}\\
\gantttitlelist[title/.append style={fill=green!15},title label font=\tiny]{5,...,12}{1}\\
\ganttbar{Task 1}{2013-05}{2013-10}\\
\ganttbar{Task 1}{2013-09}{2013-12}
Previous four lines where used to draw figure a. As you can see, first title bar is placed at top most point, second bar is placed y unit title units below top most point because \\ after first \gantttitle declares/counts a newline. And first \ganttbar is placed 2*y unit title units below top point because \\ after second title line adds a new line to the counter.
The white space between title bars and between title bar and first task bar is due to difference between y unit title and title height. This value is used to fix title node's height which by default is 0.6*y unit height.
Therefore if you don't want any white space between titles and task bars you need to use title height=1. The result will be similar to figure d. (In this case I've also declared y unit title=0.6cm.)
Figures b and c will help to understand what happens when we play with local y unit height as in your example or with \\ as in my comment. Code for figure b is:
\gantttitle[title/.append style={fill=blue!15}]{2013}{8}\\
\gantttitlelist[title/.append style={fill=green!15}, title label font=\tiny,
y unit title=.8cm]{5,...,12}{1}\\
\ganttbar{Task 1}{2013-05}{2013-10}\\
\ganttbar{Task 1}{2013-09}{2013-12}
I've kept \\ after second title but included y unit title=0.8cm in its options. As you can see this second title is placed y unit title units below origin, but now this value is local 0.8 cm instead of global 1.2 cm. This second title bar is overlapping the first one. But, because \\ is used after second title bar, the first task bar is still placed 2*y unit title global units below origin, leaving a lot of white space before first task bar.
Figure c was drawn with next code:
\gantttitle[title/.append style={fill=blue!15}]{2013}{8}\\
\gantttitlelist[title/.append style={fill=green!15},title label font=\tiny,
y unit title=.6cm]{5,...,12}{1}
\ganttbar{Task 1}{2013-05}{2013-10}\\
\ganttbar{Task 1}{2013-09}{2013-12}
There are two differences with code used in figure b. First one is y unit title=0.6cm in second title bar options. This short value makes more evident how second bar is overlapping the first one.
A second difference is missing \\ after second title bar. The result is that the code has only counted one title bar and first task bar is placed only 1.2 cm below origin, but because second title bar is misplaced due to local y unit title value the result could be sometimes acceptable. This is the reason for my comment, which worked with your particular example but won't solve the real problem.
Therefore, the solution to avoid (or reduce) white space between titles and first task is selecting a good value for title height. With title height=1, title node will have y unit title height. The default value is 0.6, therefore 0.4*y unit title is white space between title bar and task bar.
The complete code used for the example figure was:
\documentclass[tikz,border=3mm]{standalone}
\usepackage{pgfgantt}
\begin{document}
\begin{tikzpicture}
\begin{ganttchart}[ x unit = 0.30cm,
y unit chart = 1.2cm,
y unit title = 1.2cm,
hgrid={ultra thick,draw},
time slot format = isodate-yearmonth,
compress calendar,
title/.append style={opacity=.5},
% today=2014-4,
]{2013-05}{2013-12}
\gantttitle[title/.append style={fill=blue!15}]{2013}{8}\\
\gantttitlelist[title/.append style={fill=green!15},
title label font=\tiny]{5,...,12}{1}\\
\ganttbar{Task 1}{2013-05}{2013-10}\\
\ganttbar{Task 1}{2013-09}{2013-12}
\end{ganttchart}
\foreach \i in {0,...,4}
\draw[thin, red] (-.5,-\i*1.2cm)-- ++(0:3.5);
\node[above] at (1.25,0) {a)};
\begin{scope}[xshift=4cm]
\begin{ganttchart}[ x unit = 0.30cm,
y unit chart = 1.2cm,
y unit title = 1.2cm,
hgrid={ultra thick,draw},
time slot format = isodate-yearmonth,
title/.append style={opacity=.5},
compress calendar,
]{2013-05}{2013-12}
\gantttitle[title/.append style={fill=blue!15}]{2013}{8}\\
\gantttitlelist[title/.append style={fill=green!15},
title label font=\tiny,
y unit title=.8cm]{5,...,12}{1}\\
\ganttbar{Task 1}{2013-05}{2013-10}\\
\ganttbar{Task 1}{2013-09}{2013-12}
\end{ganttchart}
\foreach \i in {0,...,4}
\draw[thin, red] (-.5,-\i*1.2cm)-- ++(0:3.5);
\node[above] at (1.25,0) {b)};
\end{scope}
\begin{scope}[xshift=8cm]
\begin{ganttchart}[ x unit = 0.30cm,
y unit chart = 1.2cm,
y unit title = 1.2cm,
hgrid={ultra thick,draw},
time slot format = isodate-yearmonth,
title/.append style={opacity=.5},
compress calendar,
]{2013-05}{2013-12}
\gantttitle[title/.append style={fill=blue!15}]{2013}{8}\\
\gantttitlelist[title/.append style={fill=green!15},
title label font=\tiny,
y unit title=.6cm]{5,...,12}{1}
\ganttbar{Task 1}{2013-05}{2013-10}\\
\ganttbar{Task 1}{2013-09}{2013-12}
\end{ganttchart}
\foreach \i in {0,...,4}
\draw[thin, red] (-.5,-\i*1.2cm)-- ++(0:3.5);
\node[above] at (1.25,0) {c)};
\end{scope}
\begin{scope}[xshift=12cm]
\begin{ganttchart}[ x unit = 0.30cm,
y unit chart = 1.2cm,
y unit title = 0.6cm,
title height=1,
hgrid={ultra thick,draw},
time slot format = isodate-yearmonth,
title/.append style={opacity=.5},
compress calendar,
]{2013-05}{2013-12}
\gantttitle[title/.append style={fill=blue!15}]{2013}{8}\\
\gantttitlelist[title/.append style={fill=green!15},
title label font=\tiny]{5,...,12}{1}\\
\ganttbar{Task 1}{2013-05}{2013-10}\\
\ganttbar{Task 1}{2013-09}{2013-12}
\end{ganttchart}
\foreach \i in {0,...,4}
\draw[thin, red] (-.5,-\i*1.2cm)-- ++(0:3.5);
\node[above] at (1.25,0) {d)};
\end{scope}
\end{tikzpicture}
\end{document}
Best Answer
I just submitted pgfgantt v2.0 to CTAN. Among several other new features, this version introduces the three keys
x unit,y unit titleandy unit chart. These keys should allow you to use different line heights in the title and the remainder of the chart.