How can I write in "curly" form of g 
in an equation?
Just "g" gives me this 
[Tex/LaTex] Different symbol for g
symbols
Related Solutions
The manfnt package offers \mancube and \manimpossiblecube:
\documentclass{article}
\usepackage{manfnt}
\begin{document}
\mancube
\manimpossiblecube
\end{document}
Those commands won't directly work in math mode, but you can use a \mbox; something like this:
\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage{euler}
\usepackage{manfnt}
\begin{document}
\[
\mbox{\mancube}_{n} +(n+1)^{3}
\]
\end{document}
Of course, you could define a command if you are going to use the cube several times:
\newcommand*\cube{\mbox{\mancube}}
2017 Update: Since \mid is a relation, under normal circumstances the spaces manually inserted around \middle| should be thick \; instead of thin \,. See table on p.170 of the TEXbook.
Instead of \mid, you can use \middle command to enlarge vertical delimiter and insert spaces manually, here is an example:
\documentclass{article}
\usepackage{mathtools}
\begin{document}
\[S=\left(\frac{\sum_{i=1}^{n}X_i}{n}\;\middle|\;X_i\sim\chi^2(k)\right)\]
\[\left.\frac{\sum_{i=1}^{n}X_i+Y_i}{n}\;\middle|\;X\sim\chi^2_k\;\middle|\;Y\sim\mathcal{N}(\mu,\sigma^2)\right.\]
\end{document}
Result:
Better yet, we can make a custom command for our conveniences:
\makeatletter
\newcommand{\@giventhatstar}[2]{\left(#1\;\middle|\;#2\right)}
\newcommand{\@giventhatnostar}[3][]{#1(#2\;#1|\;#3#1)}
\newcommand{\giventhat}{\@ifstar\@giventhatstar\@giventhatnostar}
\makeatother
It works sorta like \DeclarePairedDelimiter in mathtools, for example:
\[\giventhat{f(x)=\frac{x^2}{2}}{x=1,2,3,\dotsc}\]
\[\giventhat[\big]{f(x)=\frac{x^2}{2}}{x=1,2,3,\dotsc}\]
\[\giventhat[\Big]{f(x)=\frac{x^2}{2}}{x=1,2,3,\dotsc}\]
\[\giventhat*{f(x)=\frac{x^2}{2}}{x=1,2,3,\dotsc}\]
will give you:
Best Answer
As noted in the comments the standard slanted font has a
gof this form. You can use it as\textsl{g}:The package
amstextis automatically loaded if you useamsmathormathtools. In the above example the package is not needed, but without it you will not get good results if you use the symbol as a superscript or subscript.Another caveat is that in the above code the symbol
\textsl{g}will change with the surrounding text, for example\textbf{A bold sentence containing $\textsl{g}$will have the symbol bold. That may or may not be what you want. If you need the symbol to be exactly the same regardless define a macroand use
\cginstead.