Re: Which Eeveelution should I get?
Well since you already have Gyarados, you probably don't wanna go with Vaporeon, although that's my favourite so would have been my top pick.
I've never found Umbreon to be that useful in-game because I'm more of an outright-attacker than using strategy or whatever. xD And it doesn't have very good attack or attacking moves. Then again, if you want something to absorb hits and annoy your opponents, it may be what you're looking for.
Jolteon's always good and would fit in nicely with the types you already have. It has decent moves too.
And finally I've always found Espeon to be useful. Although there are fair number of Steel/Dark types around, so it might not be that effective. It has a good moveset compared to a lot of the other eeveelutions though.
So personally I'd go with either Jolteon or Espeon. But any of them will be useful in some way. You should go with what you like best - or you could even breed your Eevee with a Ditto and get two of them. =)