queens gambit dark square bishop

Is this a good way to keep the dark square bishop from being trapped by your pawns?

This wouldn't be a queens gambit.

Queen's gambit is 2.c4.

The problem is now 2..c5 by black is going to stop any queens gambit setups from you.

A better way maybe is 1.d4 d5 2.c4 and now if they play e6 you can play Bf4.

The dark bishop is usually the KEY defender of b2 and Nc3 in many variation of Queen Pawn Game. So do not hurry to move him. 

Not in the queens gambit the bishop does not defend b2 and the knight on c3. G5 is were the bishop usually goes but f4 is also popular.

I think it does defend b2 and NC3（actually b1-a5 diagonal）in the beginning of the game until necessary setup is done, if you move dark square bishop too soon, those two weakness will be used by black.

1 d4 d5 2 Bf4  c4 , you can see Qb6 is a double attack move which give d4 pawn a hard pressure. Because your Bf4, you need to defend your d4 pawn but not to attack d5 pawn.

1 d4 d5 2 Bf4 e3,  then you can see Bb4 is a typical move which stop your c4 plan , and white's queen side advantage is gone .

Yes, after a few moves Bg5 is usually played as an attacking move but at the beginning the dark square bishop is a defender.

It's all possible actually. There is a Bg5 line, a Bf4 line, and an e3 line in which the bishop is blocked in by the pawn. In those lines it's possible that it goes to d2, when after Rb1 and b4 it can help with the minority attack, or  Bb2 after b3 is also solid. Lots of possibilities. Often things are not 'wrong' per se, although it's important to know the different strategic plans/concepts that belong to the set-up you chose. 

this is an unflexible way of entering the london system. it's perfectly playable, and also perfectly boring, but it's ok

an early assault doesnt achieve anything

op does mention "queen's gambit", though it might be a miswording, he might just mean queen's pawn game.

anyway, queen's gambit is 1. d4 d5 2. c4, and if your dark squared bishop is getting trapped while playing the queen's gambit, with the exception of a few lines it's likely that you are doing something wrong.