In the old days, the difference between the FGMP-14 and the FGMP-15 was always that the earlier model had no gravitic compensator and so must be used with Battle Dress. The high cost of the FGMP-15 reflected the cost of the gravitic compensator, which allowed an un-armoured individual to use it. There were no differences in combat performance between the two.
The costs in MongTrav for these weapons have stayed the same as in CT, but the description on page 102 of the core rules seems to deviate from the cannon description (by making PGMPs Battle Dress only and FGMPs grav assisted), and as noted by the original post calls into question the reason why anyone would buy an FGMP-15 (which appears to offer no advantage over the FGMP-14 in the MongTrav universe).
Without having seen MongTrav Mercenary, I'd suggest this is an issue the editors should look at again.