improve my atacking team!

Can you add space to your bag, and spin pokestops before you go?
Gym battles are set up to help the attacker win, your limitations are not your mons but your revives and potions.

Wow, you have some great options and already a great squad. The Jolteon will definitely help you against gyros. I have a Venusaur with the same moveset around the 2200 CP mark and I really just don’t like attacking with him. PB is so much worse than solar beam but that’s my personaly preference just in case you were thinking of the venusaur. I think the Gengar might be a good generalist to help you out overall. They are pretty strong and you may not even need to max him out.

I'm level 29, and take down L6-7 gyms with a squad much worse than yours. I would power up the Jolteon, as you will run into a lot of Gyrados, but the rest of your squad is fantastic. No need to power up any of the others.

But, if you are going to take down that many, you are going to need a LOT of revives and potions. My advice would be to go somewhere with a lot of pokestops, toss most if not all of your pokeballs, and just keep spinning.

Assume, on the conservative side, you will need 5 revives and 9 50hp potions per attack run. A L7 gym requires about 6 runs each..

So, 6x9x5=270 Revives
6x9x9=486 50hp Potions

Now, that's pretty conservative - you can probably get away with quite a bit less. But, you definately need to fill your bag with as much as you can get. That many will get you through for sure. Throw out those pokeballs - you don't have room for them.

Yes, you will want that Jolteon, and I like Discharge. Other than that, I would go for the Golem. You can use it effectively against Gyarados, Snorlax, Espeon, and Dragonite, so it's a handy all around attacker.

Rhydon with RS is a good option against normal type (Blissey and Snorlax here). You don't have any Eggys with Solar Beam ?

Your 2 Gyarados with HP will help you a lot for sure.

I don't see any mons to power up except your Jolteon. Too bad your Gengar has a bad moveset, it's a fun mon to play with and useful against Espeon.

If eevee is common where you live try to catch like 10 of them with IV above 70% and level above 25 and evolve them all. Hopefully you will get a ready to use jolteon and flareon (for the jolt anything with TS and for the flareon FS/O). I wouldn't power up any eevelution unless it is for defense or have a really hight iv/level already. The best option of you power up list is arcanine, bite is a really awesome offensive move.

You are going to need a lot more revives and potions.
Level 6/7 gyms probably turn over frequently. Focus on gyms that have been recently taken and are level 3 or below. Try to get 2 or 3 and collect. Spin spin spin.

Personally when I'm planning to attack a lot of gyms, I also use it as an excuse to clear out some space in my Pokémon storage. Personally I am a hoarder and have 1000 max storage. So I find a bunch of Pokémon I can live without, and rename them according to their type. And I put a $ in front so that when I sort by name they are in front. So for example $water would be one of the many throwaway water types I have collected in the last week.

Then you use them in battle one, then transfer. Potion saved.

It pretty much depends on what the metagame looks like there, I'd rather have a good variety of 1.5-2k "counters" than a handful of "top" attackers.

What I mean is... you want to kill gyarados? Get some Lanturns, Want to kill Dragonites? Dewong and cloyster (and lapras if you have one) are your pals, and so on, you don't need to invest tons of candy and stardust, just find good counters to common defenders, remember that in PoGo determination is the key.