I was thinking maybe it was some form of running walk, because pacing in general isn't a smooth gait, and he sure looks like he glides along without noticeable side-to-side movement. But I agree, looking at it in a still photo, it looks like a pace.
Hmmm. Any gaited horse experts out there? I know there are very fine lines between some of the gaits. Like I have even heard of a "stepping pace" where the legs move like a pace but the timing is slightly different, so the gait is smooth.
Momma can do an assortment of gaits, from a Foxtrot, to a running walk (when she is excited), to a pace (when I try to ask for a canter). So that doesn't narrow it down much! But it definitely doesn't look like a Foxtrot, that much I can determine. It seems to be his intermediate gait, which he does instead of trotting. I have also seen him canter.