OK, got this concept further after a conversation with a PR rep for Midland and some reading. Also, much of this radio tech is very new due to the FCC, in May 2017, significantly revising the rules for combination FRS/GMRS radios. (
https://en.wikipedia.org/wiki/Family_Radio_Service).
In operation, its simple to use FRS/GMRS because "frequency channels" are numbered 1, 2, 3, 4, 5, etc. You do not need to understand all this to use one, but its been very helpful to me understanding why we'd want to switch to FRS/GMRS and helpful to me when leading the trail and having people on HAM, FRS/GMRS, CB, and yelling out a window.
The power output on Midland's most powerful handhelds are limited by the FCC to 5 watts.
https://midlandusa.com/5-watt-walkie-talkie-radios/ FRS radios are limited to 2 Watts on channel 1-7 and channels 15–22 (
https://en.wikipedia.org/wiki/Family_Radio_Service). A GMRS handheld radio (mobile discussed below) can use 5 watts on the same channel 1-7 and channels 15–22. Thus, in actual practice and, of concern to us on the trail, the difference between your FRS or your GMRS handheld is not going to be that significant. I believe in all cases the FRS will still perform, again as we use them, as well or better than a CB. CB is limited to 4 watts and is AM. FRS/GMRS use FM. And FRS/GMRS are a radio where you can buy two for $80. No installation needed. Just make sure you have extra batteries or better yet get one that has a USB rechargeable battery you can plug in to your truck while driving. (I'll be testing the Midland T71 soon)
GMRS handhelds will/should technically have better range than FRS handhelds. But, if we are all using handhelds, we'll be on comparable power even between FRS and GMRS handhelds and they will all work fine for a 4x4 group within a few miles of each other. The GMRS handhelds usually have all the FRS channels built in, so if I have GMRS radios and you show up with FRS radios I can talk to you.
Now, GMRS also has available "mobile" units which can legally be up to 50 watts on channel 15-22. In Midland's case they offer mobile models in 5w, 15w, and 40w at this time. Also available are 3 & 6 db gain external antennas you'd mount as you would an antenna for HAM or CB. This antenna, which you cannot attach to a handheld is a significant part of why a "mobile" GMRS will perform better than a handheld GMRS. Unlike with many handheld ("HT") HAM radios, handheld GMRS radios and "FRS radios must use only permanently attached antennas "
https://en.wikipedia.org/wiki/Family_Radio_Service.
Dedicated GMRS mobile units installed are going to be comparable in price to a basic single band HAM installation. Handheld units cost less and require no installation - but make sure you can charge as you go or have plenty of extra batteries.
In the near future I expect to have loaner FRS/GMRS units for Rising Sun.
Now, back to what I have written about the MARS-CAP mod to "open" a HAM radio to transmit on different bands, seems that would be the best of both worlds to get your dual band (2M/70CM) HAM MARS-CAP modded ($35 by HRO) and use it for GMRS. I first used mine like this on the WY eclipse trip in 2016 to talk with people in our group with "handhelds" and have used it the past few weekends this summer the same way. Worked great. Note that it has to be on the GMRS frequensies, not FRS, and at the legal wattage. I am still researching if it is legal to use a HAM unit this way, and so far I see no issue if frquencies are GMRS and wattage is at or below 5/50 for appropriate frequencies.
@DaveInDenver, info on this?