I'm having similar problems with the same GPS module. I see many corrupted inbound messages and many garbage characters.
I got improved messages by amending the UART config:
stty -F /dev/serial0 cs7
cat /dev/serial0
After a few seconds, inbound messages were then good; full NMEA sentences and no more garbage characters.
According to the datasheet, the GPS UART should be running cs8. So it is odd that setting it to cs7 helped things. I picked up trying cs7 from an Oracle tutorial on using the same GPS unit; see link here.