ham radio emergency nets

Gmrs is regulated under part 95 where regular amateur radio is regulated under part 97. In order to be legal a gmrs transceiver has to be part 95 approved, so using your part 97 approved radios that are modified for gmrs frequencies is illegal. Not that it matters, because the FCC is  pretty much non-existent in enforcement anymore. Once in a blue moon they go after operators who are causing interference with government communications and might throw a part 95 violation in with the other charges.
If you get a gmrs license I wouldn't openly advertise that I was using a part 97 approved radio with the mars mod but it's not a big deal.