Is the Gunslinger class from Pathfinder backwards compatible to DnD 3.5 without much besides changing skills? I've got a niche they could fill, hence why I'm asking.

Similar question about other classes such as the Inquisitor, Cavalier, Magus and Witch?

Somehow, despite what I am about to say; it's really going to come down to opinion.

In general, it is perceived that if you place the two editions side by side (Pathfinder and D&D 3.5), that things (especially races) in Pathfinder are stronger than their 3.5 counterparts.

The Gunslinger and all of the other standard classes from Pathfinder can be made backwards compatible to 3.5, however; you will need to do some comparative house keeping first.

One of the most important items is Grit.  Actually this is likely to be the single biggest difficulty at all.  No class (or at least no official class) gets "per encounter" abilities in 3.5.  This was a feature of 4th edition.  Grit essentially functions as a narrative-comical way of regenerating powers and abilities.  In doing some translation to make it 3.5 viable, you would have to find a way to tone this down compared to some sort of similar class structure (perhaps comparing to a Ranger->Arcane Archer?)

Classes like the Inquisitor, Cavalier and Magus are pretty simple to handle, because they already exist in 3.5 as prestige classes.  This means building a simple character that is optimized for that prestige class, then figuring out which parts the 3.5 version never gets - take them out of the PF version.. and call it a single class and you're done.

Okay so perhaps comparing the gunslinger to a factotum or one of the ToB classes would be good? Those are per encounter classes. Thanks for giving me a place to start!

I don't see Grit as a per encounter ability, so I don't see that being as big a concern.  It is true that it provides a means to re-gain uses of their ability back, but is otherwise a per-day ability.  You can't so some kind of 'short rest' to get your use back, it has be be earned in some dangerous encounter, if not from daily renewal like most.  Combined with the fact that it is pretty limited to start with I don't suspect that it is likely to be more powerful than many of the newer inflated 3.5 classes.

I would think your biggest concern will be will you use the 3.5 rules for firearms or pathfinder ones, and if using the 3.5 ones what are the impacts to some of their deeds.  I'm not quite as familiar with the 3.5 rules, and what I am familiar with may be house-rules rather than official, but the ones I've seen have normally been different than how pathfinder handled them.