Power up for CP or type coverage?

At this point if you want to get coins to buy things and recover the investment you make on mon the king is CP, it is the only thing that really matters on the actual gym system.

If you want to stay in a gym, you need high CP mon, if you want to take down gym, just look at type coverage, but for example, no need of a level 30 bug type, with a 1k Parasect you can take down a 2.8k Exegguttor, with a 1k Cloyster you can take down a 3k Dragonite, etc...Look at weaknesses and use the mon you like, if you dodge you will always win.

So my advice is either save your dust or invest it first on high cp mon to get a return on that investment via gym coins and dust.

Wow, 9 Gya if I counted well. And you are lvl 34 at least.
From efficiency point of view, power up defenders, but only when needed to place on upper half of a gym.
You have enough attackers, I think you can beat gyms only with Gya same as I can only with Vapes :).
And besides high cp gym holders power up what you enjoy more in battles.

Sadly this game has very bad combat mechanics. Only thing that matters is CP at high lvl. It would be great if TYPE Counter damage is more like double the dps instead of pathetic 1.25...
Edit: Lets say you have 6x dragonite with high cp. You dont REALLY need anything else to attack a gym...