Many if not most handheld, (HT) radios come with pre tuned either dual or tri band antennas. Many are tuned to the 2m, 70cm and the 1.25 meter ham frequencies. They will/do work fine for GMRS. However, if you want to maximize the power output and reach of your radio on GMRS you might consider getting a GMRS specifically tuned antenna.
The GMRS band covers 462 to 467 MHz. The 70cm ham band covers 420 to 450 MHz. Usually the antennas are tuned to the middle of the band. So these 70cm stock antennas are probably tuned to 430/440 mHz. The middle of the GMRS band is roughly 464 MHz. That is not a big difference that is why they work on GMRS.
A quarter wave antenna for GMRS is only 6″ long. Half wave 12″ long. 70cm would be 6 1/2 and a hair over 12″. Again you can see not a big difference. If you were to measure this on an SWR meter the difference wouldn’t be much either. However, even the slightest difference means that not all of your power is going out the end of the antenna if you are transmitting on GMRS with a 70cm antenna.
My main HT is the Wouxun UV9Px. Awesome radio. Listed as an 8+ watt radio. I have tested it up to over 12w. It is mainly a ham radio that is modified for GMRS. It came with the usual stock dual band 2m/70cm antenna. Works perfectly fine. I also have a Comet dual band antenna which is about 15″ long and does work better than the stock antenna. But it is still not a GMRS tuned antenna. Recently I ran across a new antenna called the Melowave Bandit-G GMRS specific antenna for HT’s. So I have one coming from Buy Two Way Radios and will be testing and comparing to the others.
Given our recent efforts to establish a NEPAGMRS simplex relay network I figure that I will try to squeeze as much distance out of my HT as possible. We will see what happens.