Question that comes up often and has already is antennas.
First take-away here is your current CB antenna is itself of little value for GMRS. The two services use vastly different ranges of frequencies and have different criteria for antennas. There are no ways to practically adapt or modify a CB antenna itself to electrically work so plan on buying or building a new one.
Second is the mount might or might not be useful. Where a 3/8" bolt worked fine at CB frequencies at GMRS frequencies the bolt itself becomes an antenna. For CB you probably have something similar to this, for example.
GMRS is much higher frequency and more sensitive to abnormalities. As a result GMRS antennas often use what's known as NMO (New MOtorola) bases like this.
This was actually developed in the 1970s specifically to address issues with VHF and UHF antennas on cop cars and firetrucks. Normally these require a 3/4" hole and are best suited for mounting on sheet metal. But there's been developed all sorts of NMO mounts for lips and stuff. Leverage what hams have come up with.
More recently a version of a SO239 (similar to the connector you'll find terminating your coax on the back of a radio) appeared as antenna bases. This works OK but you usually only see this used on ham antennas since it's not great mechanically or environmentally and would probably not last long abused on municipal vehicles. These actually mount in a 5/8" hole.
Practically speaking I'm not aware any 3/8" stud antennas for UHF. There may be some but it's certainly not common. You can adapt a 3/8 stud, though. I've seen them to SO239 for sure.
This can work but it's a kludge that's going to create electrical and water leakage issues eventually. If you have a mount you really want to reuse it's better to find a mount that is more direct from the coax to the antenna base. They make NMO bases that fit 3/8" holes, for example. Assuming the mount itself has enough room for the larger diameter NMO these will work just fine.
And if you happen to have a bulkhead already terminated with a PL259 connector they make NMO bulkheads that you could use. Disconnect the cable, remove and replace the bulkheads and reconnect the cable.