mode 188 seems to work, with TTL level TX, as with:

serin2 portA.0,188,3000,toto,[WAIT("$GPRMC")]
LCDOUT $fe,1,"Found GPRMC"
PAUSE 2000

It displays "Found GPRMC".
So either it doesn't wait for satellite or the SERIN doesn't in fact find the "$GPRMC" string at all.

When I try to collect subsequent bytes in an array or in bytes, it stays stuck in SERIN... It is driving me mad as I don't have the slightest idea of what's going wrong.