For future reference and reporting GPS lock issues. To determine whether your GPS works or not, make a separated test. Take your Navio+ and RPI to an open area (field, park, backyard) where multiple satellites are visible. Connect antenna, connect to u-center software as shown in the docs and wait for 3-5 minutes (usually 40s would be enough).
Here is what your SNR levels should look like, or better:
It is very important that you place the antenna as far away from RPi as the cable allows and that you do not have any other equipment connected while performing tests (no 3G/4G modems, no cameras etc. )
@florent451 hmm… it would be a very unrealistic scenarios with two faulty boards at once. Are you sure there is no other equipment running that might create interference? Could you test it not on the copter as described above?