Hypothetical Question: level 9 Blissey Gym lowest Blissey is 2800 highest is 3150 what would you do?

I'd have been tempted to put something non-blissey-like and without common vulnerabilities (ie, not a tyranitar - they're weak to fighting) so it landed between blisseys and forced the attacker to switch pokemon between the blissey brigade and your guy, and then again afterwards.

I would put a Gyarados or Dragonite beetween them, forcing a switch from fighters, since they resist Fight moves.

If it was still a level 3 gym, I would but a mon that is easy to prestige slightly below both Blisseys and train the gym a bit or leave it to someone else to train.

I would suggest selfishly putting anything above 3150 CP to occupy the 10th spot.

Put a blissey on it of course.

Dragonite with Hurricane.

There were two times pre-Gen2 that I got into level 9 gyms filled with Snorlax. The first time I put a Snorlax. The second time all of my high CP Snorlax were already in gyms so I put in a Vaporeon with the idea that the common counters used against it would be Venusaur or Jolteon, which would not help with the rest of the gym.