Well both have their place... And you missed one
FRS is a short range UHF radio service, this is very low power, NO license required, mostly hand held hardware, Get transceivers at most any superstore and more than a few drug stores and truck stops and gas stations and even some grocery stores.
GMRS is higher power, and the most common GMRS radios live next to (And on) the FRS band in UHF (Though there are other GMRS services). The higher power gives you a bit more range but you DO need a license 80 bucks for 10 years, unlicensed operation can net you a multi-thousand dollar fine or prison time.
CB is higher power and longer range still. Used to required a license but that requirement was dropped long time ago. Many truckers and such have CB in the cab of their trucks and will discuss traffic conditions and other things.. I have had CB save me from problems twice so far,, Not so GMRS or FRS.
One time near Christmas on a road covered with ICE CB told me of a rollover a ways down the road.. I began slowing and wound up sheltering the players in a 4 car crash since I was able to SAFELY stop without damage till the police arrived and wound up taking them home after.
I do admit to needing to drive real carefully that night.
The other time was fall 2012, coming South on I-75 in KY Im about 77 miles from TN when I hear of a pair of fatal accidents long about mm-73.. Took exit 76 just as they closed the freeway, Found parking, Restful night, Good breakfast and on down the now fully cleared road. No problems.
IN both cases had I not had the CB things would have been much more stressful. (Like an hours long traffic jam in one case and possibly a crash in the other).