Getting into gyms

So far as I know, if one mon is kicked out of the gym that slot is open to take by a new mon of the same team ten minutes later. If your from an opposing team, then you'll have to take down the whole gym. And that does not necessarily guarantee you a spot.

You have two options only, prestige is over so you either take down completely an enemy gym in order to enter or you wait for a slot of your team's gym to be available.

Keep in mind that if someone attacks a gym and remove a mon from it you will have to wait 10 mins since they stop attacking before you can place your mon (of course if the gym was already from your team before).