There is also a power difference. The Garmin is a 5 watt max radio, and the Rocky is a 2 watt max. Both radios use only 0.5 watts and narrow bandwidth on channels 8-14. Do a test using one of those channels and see if the reception if the same in both directions.
You likely already have a call sign issued. Plug in your FRN number on the FCC GMRS database and see what comes up. https://wireless2.fcc.gov/UlsApp/UlsSearch/searchGmrs.jsp