Yes, if the gym threshold is still met, one of the remaining will become a gym, with certain exceptions. Showcase Pokestops will not become gyms, and property owner requests can prohibit gyms. A sponsored gym will count toward the total created through Wayfarer, so might affect your count.
As for what becomes a gym, it is not the oldest, because I have seen a newly accepted Pokestop eligible Wayspot go live as a gym.
There was a post on the old forum complaining that more pictures and likes were no longer working to create gyms, to which Niantic replied that attempting game manipulation is abuse. So use your own judgement on that. I enjoy adding photos and I thumbs up the better ones, and that has not ever been considered abuse.
They will most likely close this topic as not Wayfarer related fyi.