The Catalan is a positional gambit where white's fianchetto bishop is improved by deflecting the d5 pawn. Black can defend the pawn with b5, and this is still often playable for black, but it can overextend the queenside which can be exposed to the bishop.
White's strategy is focused on breaking up black's queenside.
Sometimes black will sac the exchange as the bishop can be more valuable.
Why should I gambit the c4-pawn in the catalan opening when i know i wont be able to defend it? Why not just play 1d4, 2nf3, 3g3 & 4bg2?
You just have to understand the piece placement, activity and plans if you play the true gambit. As an example, grabbing the whole center with e4 and placing a rook on e1 or d1 ideas. Where does your queen go?
If this is new and doesn’t make sense, I recommend immediately recapturing the pawn back and there are multiple ways to do it.
Either way, white gets piece activity, positional pressure and black compromises his pawn structure into the middle game or in some cases, misplaced minor pieces.
Why should I gambit the c4-pawn in the catalan opening when i know i wont be able to defend it? Why not just play 1d4, 2nf3, 3g3 & 4bg2?