The perfect SWR for any antenna is always 1.0:1, but an SWR up to 2.5 is acceptable if you have low loss feed line. That's true for GMRS as well.
An SWR meter for a CB might not work for the UHF frequencies of GMRS.
The antenna connectors for CB are usually the same as GMRS, but long lengths of cables that work just fine for CB can really absorb the power of at UHF frequencies, but you probably aren't running more than a few feet in your WWII Jeep.
As far as whip length, the length of your CB antenna is probably about 103 inches or something like that; that's a quarter wave whip. The wavelength of CB is 11 meters, or around 33 feet.
The wavelength of GMRS is about 64-65 centimeters or right around two feet, so a quarter wave whip would be around six inches. That would look pretty tiny compared to the long CB whip.
If I were you, I would just hook up to the antenna and test it to see how it works. Run at low power at first. Modern radios are amazingly resilient to damage from high SWR. See if anyone can hear you.
Or stay disconnected from the steel whip and just run your radio with a stubby little GMRS antenna hidden on the Jeep somewhere.