It looks like Qstarz added new messages to their NMEA protocol to provide the log fields that are not otherwise available in the NMEA protocol.
I stumbled on it in a locr post.
GPSMSG seems to come in different flavours, not sure what every field means for the moment:
$GPSMSG,0,1,2,4,5,7,0,0,0,0,0,0,0,0,0,0,0*04
-> I guess that this mesage concerns logging parameters of something like that. Seems to be valid when record number is 0.
$GPSMSG,1,T,20080419150953.000,0,50.714448,N,1.923491,W*78
$GPSMSG,2,T,20080419151004.000,0,50.714444,N,1.923476,W*74
$GPSMSG,3,T,20080419151014.000,0,50.714450,N,1.923499,W*70
$GPSMSG,4,T,20080419151024.000,0,50.714449,N,1.923494,W*71
This is clearly record number, date/time/milliseconds. position information.
BT747 has no support for that for the moment.