@Ties summed it up pretty well. A couple of things to clear up though. CB even though it’s 40 channels, only 6,19, and 21/22 (depending on the area) can become congested. And when I say can become, I mean they used to be back in the 1990’s, but CB here in the northwest is almost non existent... my Galaxy 959DX has been on for the past 18 months set to channel 19 (the unofficial “trucker channel”) with about one contact per quarter. So in other words very seldom used.
Second thing to clarify. It is illegal to use any radio not intended for GMRS on the GMRS frequencies. This includes using a Yaesu FT-4XR unlocked to transmit on the GMRS frequencies. A typical amateur radio walki-talkie puts out 1 watt on its lowest power setting, GMRS is 1/4 watt at the lower channels.