You only need one antenna for VHF/UHF and GMRS and an antenna switch. The Comet CA 4X4 antenna is made for EMCOMM work and is very wide banded. You can adjust it so you have a 1.2 to1 SWR on VHF/UHF and you will still be way under 2 to 1 swr on GMRS.
I prefer using an antenna switch, no chance of putting power back into the front ends of the other radios. It gives you almost total isolation.
If necessary to monitor VHF/UHF and GMRS at the same time, get two of the comet antennas.
They have great range, about 3.8 db gain on 2 meter and of course more on UHF.
Look at the reviews on EHam, everyone who has one loves it. I have yet to find a better mobile antenna. Larson makes a great antenna, but the comet will out talk and out receive it and it's adjustable on VHF and UHF.
They run $60.00 to $70.00 but are worth every penny, I has a Comet SSB5 and a Larson before this one and the difference is like night and day, best mobile antenna out there.
They are made tough as nails and they fold over if necessary. I use a NMO mount in the middle of the roof. The NMO mount is worth a couple S units over a mag mount due to much better grounding to the vehicle body, just scrape down to the metal on the inside of the hole so the mount bites into the metal for a 100% connection. I get fantastic signal reports as to distance and strong signal with this antenna.
It has more great reports on EHam than any other antenna I saw, and the gain really helps, it will double the output power of your radio on VHF and UHF.
73's John