you can battle trainers in selene city gym after beating the gym...

[Former user]

Idk if this is intentional, but if so... ugh

[이태은 Leiah Hartley]

I'm pretty sure you can do that in the real pokemon games too. It's only the trainers that you've already battled who can't be battled again. Besides, you don't have to battle any trainers at all in that particular gym, so what's the problem?