I feel like you meant me. 
You’re welcome! Couldn’t find any ready to go solution so far. Maybe you, I or someone else has time to test wether aquiring a GPS fix itself is possible with Flutter on Librem 5 right now?
It should be a matter of 1) following my tutorial for the virtual build machine and 2) build the flutter-geolocator linux example app and test that on the Librem 5 the same way as mentioned in the former tutorial.