Hi Alex
To me, it looks like it isnt random rubbish (problem with serial line) as the characters always appear in the same place. Maybe it is UBX messages.
Is echo disabled on the serial interface?
stty -F /dev/serial0 -a
You can try and reset the GPS using this command from /dev/ttyGSM2
AT+UGUBX="b56206040400ffff02000e61"
If this doesnt work, please send an email to sales @ ozzmaker