I believe it is part of FCC rules that radio certified as an FRS radio
cannot have a removable antenna (ie the 'bubble pack' radios). GMRS only handhelds can have a removable
handheld antenna, I believe.
Lot's of websites have antenna's what will work on GMRS frequencies, I am a happy
customer of rwiz.com.
GMRS radios (commercial grade), portable or mobile, will have external antenna capabilities. The type of connection may vary though and there are numerous antenna types that are acceptable.
FRS radios (any type) have no removable antenna and are restricted to .5 watts.
FRS/GMRS radios (bubble packs) most likely will not have removable antennas, should be .5 watts on FRS and up to 5 watts on GMRS.
To legally operate on any GMRS frequencies you must apply for a license through the FCC. The license is good for 5 years and cost $80 (unless the cost has changed recently).
Big difference in FRS (only) radios, FRS/GMRS (bubble packs) radios, and GMRS (commercial grade) radios. If you’re talking about FRS radios, then NO you can not attach an external antenna.
IMPORTANT NOTE: There are some who feel that it is unnecessary or a waste of time to legally obtain an FCC GMRS license and freely advocate that and may inform you that you may never get caught, which may be the case but why risk it?