FRS & GMRS frequencies have been preset by the governing bodies where these radios are licensed for use. They consist of Ch 1 - 14 for FRS or Family Radios Service (the new "CB" radio) and Ch 15 - 22 for GMRS or General Mobile radio Service. GMRS frequencies may be repeated
5 Mhz higher - the repeater input of channel 22 (462.750 Mhz) may be heard if close to the transmitter on Ch 22, but if a repeater has been set up to repeat signals in range on Ch 22, they would also be heard on 467.750 Mhz. The repeater will likely have one or more of the following: a high vantage point such as a hill or mountain top, may increased output power of up to 50 watts, or have an antenna with gain properties. Any one or combination of any of these benefits, will result in a significantly increased area of coverage.
Frequency in MHz
It is important to note that only Ch 1 - 14 may be used without a license. Transmitting on any of the GMRS frequencies (Ch 15 - 22) shown in red in the chart above, requires an FCC license if used in the U.S.A. Other countries may or may not have laws for licensing the use of any or all these frequencies, so check before
transmitting. Modifying an FRS / GMRS radio is a violation of FCC rules and if caught; can result in fines, prison or both.
If you'd like to be able to tune actual frequencies - and not be limited to specific channels, you ought to look into amateur ("ham") radio. While not in the FRS or GMRS bands, there are literally hundreds of thousands
of frequencies to use once licensed. The ARRL
(American Radio Relay League) has a great deal of info about ham radio.
I hope this helps & good luck! Please rate my reply - thanks!