In the US you need a GMRS license from the Federal Communications Commission
(FCC) in order to transmit on GMRS frequencies. The GMRS license is good for the whole family,
though, with an unlimited amount of radios.
In Canada you don't need a GMRS license, however, official legal output is supposedly limited to 2
watts (although if you look on Canadian sites offering GMRS units for sale, they offer the same specs as the US
versions). Also, repeaters are forbidden for GMRS use in Canada.
Some GMRS walkie-talkies have a "power boost" button (or something similarly named, I have this option on my
Uniden handie-talkies), however most of the consumer grade GMRS walkie-talkies will only put out a couple watts or
maybe a bit more. It's almost impossible to get an actual rating from the manufacturers, as they seem bent on
telling consumers that their little handies will reach up to 35 miles, instead of giving us some sort of indication
what the output power is to the antenna. Still, even with these 2 or 3 watt handies, in the US you will need a GMRS
license. Since I operate an Icom UHF mobile capable of 45 watts on GMRS frequencies and the Icom mobile rig is
repeater capable, I think the GMRS license is certainly a fair regulation.
For complete FCC licensing info, go to the FCC GMRS home page. There, you will find an online form to get a GMRS License.
For possible changes to the future of GMRS, see this page:
History and Future of GMRS