Are you aware there are twenty (20) different guantlet romsets currently in mame, not even counting guantlet II or latter games? Mame didn't have all of these sets from the beginning, and some of the first (incorrect) sets included ROMs from different set or bad dumps (bad copies). IIRC, a rom that wasn't dumped nor needed earier, was just semi recently correctly emulated are is now needed. Also, I believe the gauntlet parent romset was changed to the current one, as it was added to mame later but is the "best" parent set. Also, romset = the zip file, rom = one of the files in the zip file; even though "rom" is often used when "romset" should be.
If mame had to keep backward compatibility to all versions of mame, mame won't go forward. Why keep compatible with a known bad dump rom if the correct dump is available? Why keep compatible with sets that never existed in the arcades, but were the best guess of at the time it was added to mame? Why stay compatible with a set that's missing a rom that wasn't needed before because that part wasn't emulated yet, but is now emulated and the rom is needed for emulation? On the other end, a new game or romset couldn't be added if the dev wasn't absolutely sure it's complete and correct (and even they might be wrong).
Backward compatibility is a limiting crutch that other emulators have had and thus died by.
A couple solutions for your problem with your romsets:
Use different versions of mame for different games, and a frontend that works with them.
Use a rom manager, and maybe a rollback set.
edit: You can look up gauntlet over at
maws and see if your missing roms are in any of the current clones, as well as when the sets were changed.