Max CP and skill DPS Balance

I think a better skill is way more important in actual combat. A pokemon's CP, depending on its IVs, usually doesn't differ too much within its own type. That being said, ideally you want to have pokemons with best skill set and high IVs. That makes them best within their type.

I've found that strictly speaking my highest cp pokemon have the most effect in gym battles, even packing not very effective moves (and this makes enough sense as this game drastically scales down weakness, resistance, and immunity). Technically, "CP" is an aggregate stat, it's a generic rating summarizing a pokemon's Go attack, defense, and hp stats, which are themselves aggregates of the the stats in the main games.

"DPS" might be slightly overblown on this site, because what really matters is what you're capable of performing. If move "a" does 10 damage but has a .45 second cooldown, and move "b" does 12 damage but has a .55 second cooldown, what may matter in real time play is that you can usually only swipe twice with both moves before performing a hopefully successful dodge. As well, for the charge moves, it may matter when a move can cleanly knock out an opponent, or if it gives expanded "coverage" (though the need for coverage is slightly decreased because of the aforementioned down-scaled weakness, resistance, and immunity).

All that said, taking on gyms is not necessarily tough, as the attacker has a lot of advantages (high level players in gyms still have stronger pokemon, and high level gyms can have more pokemon on the roster than challengers, but these are the only cases where gym controllers have advantages).

I heard on a video that when a pokemon is defending a Gym it gets its HP doubled and that's why Lapras, Vaporeon or Snorlax are so popular.

CP is directly related to HP (stamina) and other stats so it may influence more at defense since it would get doubled the stat (which is higher already).

I cannot confirm that but i think it helps you a bit.