Where to put white's black squared bishop in the queens gambit?

My knowledge of openings is limited but I apply the golden rules (fight for centre, develop, castle and connect rooks). I always play the queens gambit with white to get familiar with at least one opening system. And i have always played my black squared bishop at d2.

This is the Slav, not the Queen's Gambit Declined.  Different openings, different answers.

In your example, Black's second move is dubious at best!  White should take on d5.

To answer your question, it must be broken in 3 parts:

A) 1.d4 d5 2.c4 Nf6?! 3.cxd5! Nxd5 4.Nf3! With e4 coming in the not so distant future (depends on Black).  Advantage White!

B) 1.d4 d5 2.c4 c6 (Slav Defense) 3.Nf3 Nf6 and now you can play a solid but drawish exchange line (4.cxd5 cxd5 5.Nc3 Nc6 and now the Bishop goes to f4!  NOT g5 as the Black e-pawn has not moved, so the knight is not pinned) or you can play 4.Nc3 where if 4...dxc4, then 5.a4 and 6.e3 or 6.Ne5 and that Bishop on c1 stays home for a while.  If 4...e6, you can play 5.Bg5 (Anti-Meran) or 5.e3 (Allowing the Meran and again that Bishop stays home and goes to b2 often)

C) 1 d4 d5 2.c4 e6 3.Nc3 Nf6 and here, you do not want the Bishop inside the pawn chain.  So 4.Bg5 followed by 5.e3 and 6.Nf3 in most cases.

Hope this helps.

1. d4 d5 2. c4 Nf6? is the Marshall Defence, it's a crappy opening that allows white a clear advantage without any counter to it. 

The dark-squared bishop is bad in queen pawn openings. Sometimes white moves it to g5 and would like to exchange for black's opposing bishop. Bf4 is reasonable also, but white must be able to deal with Nh5 or an attack on the queenside such as Qb6. 

4. e3 is probably the objectively best move in the given position.