Hello Hermann,
I'm not sure where this Google Fonts come from if you are using a local font.
Anyway, this is out of the scope of support and in my personal opinion you really shouldn't care about it.
Yes, I do understand that you want to be strict, but the whole world uses Google Fonts without any contracts and whatever.
If you want more information you might want to
open a new issue on Github and the Gantry 5 developers will reply.
You might also want to ask your question in the
Gantry 5 chatroom on Gitter.