In that video interview he is talking about the back gate while pointing towards the front gate.
Also, later in the video,
Tracy said that Trayvon came in the front gate (1:03-7).
I have always thought that it was probably because he was coming off the freeway on his visits and used the back gate as his primary entrance. The proximity of the Green TH to the gate is much closer and ease of access to/from the freeway.
It looks like the way to get to RATL from the rest of the world, is to take I-4 to Highway 417, exit to Rinehart Rd., and go to Oregon Ave., which goes by the front (N) gate of the complex.
The complex has a 15 mph speed limit. You can see one of the signs
in the reenactment.
If I were going to or from a place inside the complex, I would want to use the entrance closest to that place, regardless of which way I was going or coming outside the complex. So I think Brandy and Tracy would have mainly used the back (SE) gate.
The gate is near the south end of the east wall. Brandy's unit faces east, the same way as the gate, which is just around the corner from it. Going out the front gate on foot, it would be natural to exit the unit by the front door. By car, which is probably how the adults usually went, the same would be true because the cars are parked in front of the units.
Early in the video (0:35), Tracy said that Trayvon came in by 'that back gate', gesturing toward the N gate, as you mentioned. In the next sentence, he said Trayvon 'was going to the back of the house'.
Later in the video (1:03-7), Tracy said this:
When he said the clubhouse, I knew for a fact that Trayvon came in that, the front gate.
To me it sounds like Tracy might have been about to repeat 'that back gate', only at the last second recalling that the N gate is officially the front gate.