Hi and Welcome!! So exiting to be getting your own horse again after all these years!!!
A little background...I too rode hunter jumpers in my youth & teens. After college, kiddos and a move, I was able to start riding again and haven't looked back. Went into Dressage for many years, until divorce stooped that (very expensive) world.
Now I just putter around on my property practicing basic dressage and go trail riding, often alone.
Based on what you have said, IMO you will love riding the trails and doing some endurance. It is a lot of fun
Now the horses; I have a RMHA gelding and I love him so much. He is a bit of a handful at times, but smooth as silk. He is not however, suited to endurance as just not fast enough. The gelding you are looking at, Does have endurance training, so I think he would be a great choice for you! It is weird at times to not trot, but your prospect may trot also, some do. Definitely do xrays of his feet though, to make sure every thing is ok. They are prone to metabolic issues, as are some other breeds.
The mare you are looking at, I am not sure will suit your endurance goals. Her back is very concerning to me, as to saddle fit but more importantly huge withers could mean kissing spine. So you would need to get xrays of her back in any PPE. The other issue is she is a draft cross, and although some do well, some do not. They also tend to have metabolic issues, and IME do not tend to remain sound long term. I had an Arab/Percheron cross gelding who I loved so much (he was my last Dressage horse) but he had to be pts at age 14 because his joints were crumbling. Another cross I know, shown up to GP had to be pts at 17 for severe coffin bone rotation. I could name more, but bottom line is they are more likely to break down sooner.
I would choose the RM gelding. He is experienced and eager. He may need a bit of management feed wise, but overall he sounds the best. Don't worry about not knowing much about gaited horses, they are pretty easy to understand. Main thing is no or very little lunging or small circles, as their movement is lateral. If you continue to take some lessons, you can get in trotting time!
My gelding soles get thin in lots of rain, so he wears shoes and pads in the winter. We just pulled his shoes yesterday, and his feet look great. Might be all this gelding needs