GMRS radios transmit 5 watts instead of 2 watts used on a FRS radios. To use a GMRS radio legally, you need a GMRS licence from the FCC. This is easy enough to do and does not require a test. You just pay the fee and agree to the terms and you and your immediate family are good for 10 years.
You need a Username and Login
If you don't have one, register for one here: https://apps.fcc.gov/cores/userLogin.do
Click on Register
Fill in all the information and Create Account
You need a FCC Registration Number (FRN)
In CORES, use "Manage Existing FRNs" the Manage FRNs" to see your FRN
You can search for it here by looking up your callsign or name
Tutorials are here
Commission Registration System (CORES): https://apps.fcc.gov/cores/userHome.do
Go to License Manager
Log in with your FCC Registration Number (FRN) and password
Click "Apply for a New License" on left side menu
Select "ZA - General Mobile Radio Service (GMRS)" on the drop down list
Complete registration
Review
"Continue to Certify"
Sign application by typing your name in the boxes
Payment
ULS will calculate your fee
Click "Continue For Payment Options"
Pay fee
Download Remittance
Wait for application to be processed
Don't expect a confirmation email
Plan to wait at least one business day
View and Download Electronic Authorization
Go to License Manager
If your Electronic Authorization is approved, download it here
Congratulations, you can now legally use your GMRS Radio within the confines of US Law