A half wave antenna is normally fed in the middle with a balun or some type of matching device (gamma match for a yagi driven element for example). Simply cutting a 1/4 wave for 2-meters down to a half wave on the GMRS frequency would present a horrible match at the base of the NMO mount. You would be much better off cutting the antenna down to a 1/4 wave on the GMRS transmit frequency center (about 6-1/4"). The next best thing would be a 5/8 wave antenna, but this would take a matching coil at the base. Both designs require a ground plane, for instance, a permanent NMO mount through a metal roof.
You did mention that you wanted to put the antenna on your Jeep window frame, with no ground plane. They do make no ground plane antennas which a lot of people in my area are using on tractors with fiberglass cabs. They are a compromise, however, because there's nothing to stop the RF from traveling back down the coax (the reason mounting through a metal roof is best). When RF travels back down the coax, it distorts the radiation pattern and you've got you performance basically resembling a hand-held radio with a little more power. You can put ferrite chokes on the coax near the base of the antenna to help a little.
Here's a link to a Maxrad no ground plane antenna that's pretty popular around here:
https://www.arcantenna.com/products/muf4505ngp-pctel-base-loaded-chrome-coil-rugged-450-470-mhz-mates-with-1-1-8-18-thread-mounts-including-3-4-mounts-optional-spring
Another option is to use a through-glass antenna which mounts on the windshield. Here's an example:
https://www.arcantenna.com/products/pulse-larsen-kg450ud-glass-mounted-whip-antenna?currency=USD&variant=32297166831747
Either one of these would be suitable for GMRS operation.
Best of luck,
Derek, WRMD298 - N4DBM
