Another French resident who has never heard of Google Fi as well. But then again, I generally eschew Google products so perhaps I'm somewhat out of the loop.
Google Fi is available only to Americans. I'm not surprised people who live outside of the US have never heard of it, just like most US residents have never heard of Orange or SFR.
Like the other US mobile providers, Google Fi has no service in France or anywhere outside the US. Instead, they have roaming partners (like Orange or SFR - not sure which one, probably one or the other, maybe both). I'd guess any issue with data in Europe with Google Fi has to do with the network it is connected to, but I've never used it.
Sometimes you need to set the Access Point Name (APN) to get mobile data to work when roaming. Here is how one person set his (Google Fi used to be called Project Fi - they are the same thing):
https://www.whistleout.com/CellPhones/Faqs/115014797887-Project-Fi-APN-Settings
Every phone is a little different. Some phones seem to pick up the APN automatically and other need to have it manually set. My Moto E2 and E4 phones automatically picked up the APN for my Dutch Vodafone SIM wherever I've used it in Europe. Some more expensive phones and newer phones do not so the APN must be set manually (which is surprising - but that's what people who had problems say they had to do). I have a Moto X4 now but haven't used it overseas yet.