A Marshall JTM45 is essentially a Fender Bassman, and Clapton didn't do too bad with that setup in the Bluesbreakers? Also, as all Marshalls are basically derivatives of the Fender Bassman, a Gibson/Fender combination is close to a Gibson/Marshall combination. I think that works OK, don't you ;)
Lots of folk use Gibson / Fender combinations:
Gibson Les Paul Standard and Fender Deluxe = Billy Gibbons
Gibson ES335 and Fender Princeton Reverb = Larry Carlton
Gibson Les Paul Custom and Fender Twin = Keith Richards
Gibson ES355 and Fender Dual Showman Reverb = Chuck Berry
Gibson Les Paul Special DC and Fender Deluxe clone = Twinfan ;)
The list goes on.......