How do you stack gyms?

Absolutely not Gyarados or Ttar. Double weaknesses.

Any combination of, but in no particular order Dragonite, gyarados, togekiss, metagross, venasaur, Tyranitar.

Milotic used in place of gyarados at times and driflbim are also what I use, but arent as powered up yet. We dont use blissey/chan/lax as a way of making people whose gym parties are 50% machamp need to remake their team for additional time stall.

I don't really care what's in a gym, I always have a "default" team that starts with a Machamp because 90+% of the time it's a Blissey first. There's a Raikou, Groudon, Dragonite on it.
There's nothing on gyms that makes me think "oh no, a high level Pokemon, I better prepare x, y, z to take it down!"
The gyms are a joke.
If Niantic wants them to be more difficult, double the defense and attack of all gym monsters, that would be more interesting.
Oh and get rid of golden berries and make it some kind of motivation system where the Pokemon have to be beat "x amount of times".

Nothing puts me off taking a gym if I've got the time... Obviously you're dependent on what others put in, but having to change attackers with a few bulky Pokemon in the mix is the best you can hope for. I don't like Ttar as a defender and you might want to mix up motivation decay a bit more. Maybe Metagross, Blissey, Milotic, Umbreon, Snorlax, Gyrados/Dragonite with outrage

Anything that stops Machamp. I fought the hardest gym the other day. Blissey, gardevoir, Togekiss, Metagross, Driftbloom, and snorlax. That was fun.

Ttar is pretty bad as defender. Gary is good, i dont think many player use Electric in gyms. I havent and most of my mates.

My fav defenders are Blissey, BS Snorlax, Milotic, DC Dragonite, Metagross, and Chansey bc motivation decay.
Slowdudes are also good because they resist BOTH Champs and Metagross, the most common attackers atm.
Other mons I like are Exeggutor (legacy) and K-Muk

The right way to stack gyms is fight-weak followed by fight-resisting, for example:
Blissey -> D'nite -> Snorlax -> Slowbro -> Chansey -> Milotic

Though D'nite has double weakness, it remains a top tier defender.
Gardevoir imho was the best Fighting-resistor, but then MM-Metagross came out and now Gardevoir is a joke.

Your goal is to force attackers to switch their mons while fisting their way through your defenders. If attackers did not switch they would faint out, that's what you are looking for

The most cruel lineup that I can come up with for competitive areas:
1. Blissey (Zen Headbutt + Dazzling Gleam)
2. Gyarados (Waterfall + Hydro Pump)
3. Steelix (Dragon Tail + Crunch)
4. Dragonite (Dragon Tail + Outrage)
5. Metagross (Zen Headbutt + Meteor Mash)
6. Milotic (Dragon Tail + Surf)

Blissey, Togekiss/Clefable, Chansey, Metagross, Snorlax, Slaking.
That’s how we do it every time. But, there’s a spoofer who always pops in a Ttar.

I don’t think there’s any good gym defender that resists Fighting, Electric and Steel. The only type combination I can think of is Psychic/Electric, but I can’t even think of a Pokemon with that type.