[Tex/LaTex] Roboto font for greek letters

Question

I am trying to use roboto font documentwise. while the roman fonts are working fine, greek fonts are giving problem.

\documentclass[12pt]{beamer}
\usepackage[sfdefault]{roboto}
\usepackage{amsmath}
%\usepackage{mathspec}
%\setmathsfont(Greek)[Lowercase=Regular,Scale=0.76]{roboto}
\begin{document}
$\gamma = G$
\end{document}

Any solution please?

**UPDATE 1 ** Henri's solution is giving me error(using xelatex):

kpathsea: Running mktexmf Roboto
! I can't find file `Roboto'.
<*> ...:=ljfour; mag:=1; nonstopmode; input Roboto

Please type another input file name
! Emergency stop.
<*> ...:=ljfour; mag:=1; nonstopmode; input Roboto

Transcript written on mfput.log.
grep: Roboto.log: No such file or directory
mktextfm: `mf-nowin -progname=mf \mode:=ljfour; mag:=1; nonstopmode; input Roboto' failed to make Roboto.tfm.
kpathsea: Appending font creation commands to missfont.log.

While using together with fontspec, I am getting a clash of packages

Answer 1

Just use mathspec.

\documentclass[12pt]{beamer}
\usepackage{amsmath}
\usepackage{mathspec}
\setallmainfonts(Digits,Latin,Greek)[Scale=0.76]{Roboto}
\setallsansfonts[Scale=0.76]{Roboto}
\begin{document}

\begin{frame}
  Hello world! $\gamma = G$
  \[ a^2 + b^2 = c^2 \]
\end{frame}

\end{document}