I definitely think Gilmour can be a bottom pair guy. His defending leaves a lot to be desired, but his skating ability always keeps him in the fight.
Don't kill me for the comparison, but I think he could defensively evolve like an Erik Karlsson. Karlsson was a pretty poor defender when he first entered the league, but his insane skating ability always kept him contesting the play in some way, shape, or form. Over time, he was able to improve and is now a very good defensive player. Karlsson, of course, has more talent in one pinky than Gilmour does in his entire body, but I could see Gilmour turning into a 3rd pair version, stylistically. That is assuming everything goes right for him. He's already 24, though, so maybe he is what he is right now and everything I've just said is bunk. Who knows. I really like his play style though. Him and Pionk aren't allergic to shooting the puck like almost everyone else on our roster.