Doesn't seem like a comparison that fair. Using a 2 watt UHF with an integrated antenna inside a vehicle against a 4 watt CB with an external antenna. Hardly apples to apples. More like apples to fish sticks.
Most UHF HTs claim 5 watts but put out more like 4. So if you put a GMRS on a small external antenna it would be much more of a fair comparison.