If you do NOT have a GMRS license:
You may use the FRS only channels.
You may use the shared FRS/GMRS channels… but ONLY on LOW power.
**IF, however the shared FRS/GMRS channels are NOT narrow band like FRS rules require, then you CANNOT use those channels at ALL. Consult the radio manual to make this determination. Most consumer-grade stuff is set to narrow band.
You CANNOT use the GMRS only channels at all.
If you get a GMRS license, then you can use the FRS/GMRS channels on any power setting you want and the GMRS/only channels. You may continue to use the FRS only channels also.