In my humble opinion, it's not an issue of ham vs GMRS. Or what ham has over GMRS. There are advantages to both. GMRS doesn't require a test, just your $35 and a promise to obey the FCC rules. And the license covers family members, up to a point. But the frequencies are limited, and channelized. Ham radio requires a test, the same $35, and only covers one person. There are many more frequencies available with UHF ham radio. Both ham and GMRS can operate simplex or through repeaters. You'll probably find more repeaters on ham radio. I use both, since I have ham radio friends who communicate with me, and I use GMRS to communicate with my non-ham wife through a repeater if I'm out somewhere with no cell coverage. My suggestion is to use both and don't fall into the "one is better than the other" trap.
Also, there are a few mobile ham transceivers out there that do HF, VHF and UHF. They're not cheap, and you may end up rolling the dice in the used market to find them. The Icom IC-706MkIIG, IC-7000 and IC-7100, the Yaesu FT-857, and others.