What is up with the Gyms?

doesnt really matter you could throw in a pidgey or blissy youll still get 50 max

Blissey, Snorlax, Chansey, Lapras are good options. Granbull / wigglytuff with fairy charge attacks even hou respect attackers to attacks with Dragonite and the above mentioned are not available.

Slowbro, Slowking are also good alternatieve.

I still will drop in my best available defender.

Blissey

I put my best defender in high traffic gyms where I can get support. Most other gyms I just throw in a le vel 30 Pokemon I was going to transfer, like a Slowbro and Tentacruel I evolved, or just throw something else I was going to transfer, so I I don't spend potions and revives.

I like to use my non-popular favorites like Marowak, Tangela, and Starmie. I'm usually not in more than 3 to 4 gyms at a given time. I use these because no one else in my area does and I can get to them fast. Mainly for situations when your riding passenger and at a red light and see an open spot in a gym. The pokemon I mentioned earlier I've never seen or are rarely in gyms, so I know they are sure thing and I don't have to go looking for one.

We talk about putting something in to counter generalist sweepers (Dragonite)--maybe PR Donphan or Dragontail Steelix. That is true for damaging above average players that would sweep with a Dragonite. Below average players might use their new legendary birds, maybe sneak a water gun + stone edge Quagsire to counter Moltres sweepers

I just go to 2-3 gyms and put in something with low HP like Cloyster and hope they last a few hours but not a few days.

All of my old Prestigers above 1350CP are now gym defenders. They seem to be able to stay in gyms between 4 and 10 hours.

If I'm angling to get gold in a gym I frequently pass or raid often, I put the good stuff in. After that, it depends... if there's a free spot in a gym full of crap, I'll toss in a mon I'm going to transfer afterwards. A few spot in a decently defended gym, a decent defender goes in too. I take over a gym nobody touches, decent. I take over a gym that lasts an hour at best, something meh. It all depends on the situation and, lately with raids requiring most of my potions, I've been more willing to hold back my best guys.

I've renamed a lot of former prestigers / evolves etc "-". Sorting by name puts them at the top, so I can grab and go when I just want a throwaway.
I'll drop Blisey or Snorlax into a gym I've just taken, especialy if it is in other team's territory.
I was thinking about using disposable attackers too, but prefer the speed of using Dragonite or Ttar to roll through. Some ppl have posted here about using revived (but not potioned) Vapes as their generalist gym sweepers, I haven't tried that.