Not sure if I have a ridden GPX of the “official” Gap route as every time I ride it tends to be a bit of a variant.
Here’s one I did a couple of weeks back for Wild Rides who sometimes posts here. A more interesting climb and descent than the normal route from Talybont. You’d probably not want to go all the way down into Ponsticill – that’s where these clients were staying – but you could turn right after Dolygaer to head back up towards the Gap.
https://www.strava.com/activities/531904053
Edit to add – not sure you’d make any friends trying to ride up, or riding back down, from Pen y Fan or Fan y Big. Neither are bridleways, both busy with walkers most of the time, and there’ve been lots of problems with erosion all along the escarpment there (caused by walkers, obvs, not MTBers, but illicit riding is not likely to go down well and shouldn’t be encouraged IMHO).