For reasons that I find very mysterious, Unicode has the full range of Greek in sans serif bold, upright and italic, but it doesn't cover sans serif Greek in medium weight; to wit, there are
MATHEMATICAL SANS-SERIF BOLD CAPITAL GAMMA (U+1D758)
MATHEMATICAL SANS-SERIF BOLD ITALIC CAPITAL GAMMA (U+1D792)
and the other Greek letters (upper and lower case), but no medium weight ones.
So it's not surprising that \symsf{\Gamma}
(or \mathsf{\Gamma}
in an older version of unicode-math
) gives no different symbol: there's none that satisfies the request.
You can get through with \textsf
:
\documentclass{article}
\usepackage{amsmath}
\usepackage{unicode-math}
\begin{document}
$\Gamma\ne\textsf{\upshape Γ}\ne\symbfsf{\Gamma}$
$Γ\ne\textsf{\upshape Γ}\ne\symbfsf{Γ}$
\end{document}

A kludge, I know, but I can't offer any better. Of course the sans serif font defined in the document should support Greek.
:(
– egreg Sep 10 '15 at 20:24