Man kann die Schriftart `academicons` [`academicons`](https://jpswalsh.github.io/academicons/) benutzen. Diese hat ein Zeichen für ResearchGate.
\documentclass{moderncv}
\usepackage{fontspec}
\newfontfamily\academicons{academicons.ttf}
\DeclareRobustCommand\researchgatesocialsymbol{%
{\academicons ^^^^e902}}
\moderncvtheme{classic}
\name{}{}
\social[researchgate][www.researchgate.net]{researchgate-Name}
\begin{document}
\makecvtitle
\end{document}
![alt text][1]
Hat man diese Schriftart nicht zur Verfügung oder will sie nicht benutzen kopiert man einfach den SVG-Pfad für den Buchstaben und malt ihn mit TikZ.
\documentclass{moderncv}
\usepackage{tikz}
\usetikzlibrary{svg.path}
% from https://jpswalsh.github.io/academicons/
\DeclareRobustCommand\researchgatesocialsymbol{%
\tikz\fill svg[scale=.001ex] {%
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\moderncvtheme{classic}
\name{}{}
\social[researchgate][www.researchgate.net]{researchgate-Name}
\begin{document}
\makecvtitle
\end{document}
![alt text][2]
[1]: http://texwelt.de/wissen/upfiles/test_282.png
[2]: http://texwelt.de/wissen/upfiles/test_283.png