FRS and GMRS radios share the first 22 chanels. Chanels 1-7 are limited to 2 watts for FRS and 5 watts for GMRS. Channels 8-14 are limited to .5 watts and use FM-Narrow. Channels 15-22 are limited to 2 watts for FRS and 50 watts for GMRS.
The repeater channels share the same receive frequencies as channels 15-22, but automatically shift to a 5 MHz higher frequency when they transmit. Repeaters generally require a CTCSS or DCS tone to activate the repeater. For open repeaters, the transmit (TX) tone (CTCSS) is typically 141.3 Hz. Contact the repeater owner for permission to use their repeater and obtain the tone required. A good source for repeaters is www.mygrms.com .
Your should check the programming of your FRS or GMRS radio. Different manufacturers can use their own channel numbers and tones making use more complicated. Channel 1 on one manufacturer's radio might be channel 15 on anothers. Each channel may also be preprogrammed with tone requirements inhibiting your ability to talk to another radio from a different manufacturer. For broad compatibility purposes, you should program your radio according to the South County GMRS frequencies list and remove all tones except those required by repeaters. This will insure your channels match everyone elses and you don't have any tones programmed inhibiting your ability to hear others.