There used to be an exemption in 47 CFR Part 95 that would allow GMRS on Part 90 approved radios but I haven't been able to
There does seem to be some room for interpretation in 95.335(a). Note that 95.735 refers to Radio Control, so I didn't post it here.
95.335 Operation of non-certified transmitters prohibited.
Except as provided in paragraph (a) of this section, no person shall operate a transmitter in any Personal Radio Service unless it is a certified transmitter;
that is, a transmitter of a type which has obtained a grant of equipment certification for that service, pursuant to part 2, subpart J of this chapter. Use of
a transmitter that is not FCC-certified voids the user's authority to operate that station. See sections 302(a), (b), and (e) of the Communications
Act ( 47 U.S.C. 302(a), (b), and (e)).
(a)Exceptions. Under certain exceptions, non-certified Personal Radio Service transmitters, or transmitters certified for use in the land mobile radio
services may be operated. Any such exceptions applicable to stations in a Personal Radio Service are set forth in the subpart governing that
specific service. See e.g., §§ 95.735 and 95.1735.
(b)Revoked or withdrawn certification. In the event that the FCC revokes or withdraws a grant of equipment certification for a type of Personal Radio
Service transmitter, existing transmitters already in service may continue to be operated unless and until the FCC determines otherwise and
gives Public Notice of that decision.
(c)Grantee permissible modifications. Only the grantee of the equipment certification may modify the design of a certified Personal Radio Service
transmitter type, and then only pursuant to and in full compliance with the requirements and procedures for permissible changes and modifications in
part 2 of this chapter. See §§ 2.932 and 2.1043 of this chapter.
But 95.1735 was a proposed rule that doesn't seem to ever have been adopted.
95.1735. GMRS equipment certification exception.
Notwithstanding the general requirement of § 95.335, a Part 90 certified transmitter programmed such that it fully complies with
the applicable technical requirements in this subpart may be operated in the GMRS band. This exception does not apply
for transmitter types where anycontrol, switch or other type of adjustment—which, when manipulated,
can result in a violation of the rules — is accessible to the user.
I wonder if the revision of FRS/GMRS and the subsequent rulings this past September back burnered that exception. Maybe it will be revisited when the FCC gets a handle on the mess. Part 90 radios I think are usually rubber stamped for Part 95, Subpart E approval if the manufacturer requests and submits for it. The technical requires for Part 90 exceed those of GMRS. But AFAIK there's nothing official that lets anyone use Part 90 radios on GMRS, so maybe it's a situation of seeking forgiveness rather than asking for permission.