I think the core argument comes down to the fact the FCC has described the channel uses for GMRS repeater (and other main) channels as:
having an authorized bandwidth of 20 kHz and a peak frequency deviation not exceed ± 5 kHz.
However, the regulations go on describe
§ 95.1771 GMRS emission types.
Each GMRS transmitter type must be designed to satisfy the emission capability rules in this section. Operation of GMRS stations must also be in compliance with these rules.
(a) Each GMRS transmitter type must have the capability to transmit F3E or G3E emissions.
(b) Only emission types A1D, F1D, G1D, H1D, J1D, R1D, A3E, F3E, G3E, H3E, J3E, R3E, F2D, and G2D are authorized for use in the GMRS.
In practical terms the only two useful types are F3E (FM Voice) and G3E (Phase Modulated Voice), which are essentially the same to the receiver. The other modes allow for AM and SSB and limited data, as permitted in the regulations. Personally, I have never heard on anyone operating AM or SSB on GMRS, but I guess it could happen.
So, the FCC has defined maximum channel bandwidth, deviation and transmission types. But, here is the issue, they have not mandated those. So, it is totally permissible to operate narrow band (NB) on any GMRS channel.
Now, in practical marketing terms, any company that only markets NB units for GMRS is obviously not going to push or promote wide band (WB) usage. However, since some GMRS channels are designated as NB, it is unlikely a WB only radio would be marketed as it could not transmit on the interstitial channels.
Which brings us back to the two experts.
...We have two members, each an "expert" with FRS/GMRS rules and equipment. The problem is one says that GMRS is Narrow Band while the other says it Wide Band. ...
In fact, if the statements of the two experts have been reported accurately, the answer should logically be neither or both. If by "is" they mean can be used as, they are both correct. However, if they mean is mandated as, then both are wrong.
In fact, I would now say a true expert would not have defined GMRS in such a way and would have offered a better description as is provided by Part95E and the responses to the original question. So, in my mind, the answer is neither expert is correct.