In regular queen's gambit, you are happy to give away the c4 pawn because your light squared bishop is staring at the square all game and you will probably be able to eventually retrieve it. In the Catalan opening, your bishop has moved to g2, where it cannot exert pressure on c4. What do you do if your opponent then accepts the c4 pawn?
The primary idea in the Catalan (specifically the open Catalan), is to use the time that black has wasted capturing a pawn to develop quicker and squeeze a small advantage out of the middle game through more active, and optimally placed pieces. The pawn is difficult for black to hold onto if white chooses to pursue material equality, but that style of play is not always compatible with the themes in the Catalan.
To answer your question about regaining the pawn, Qc2 is usually sufficient. Black can defend the pawn but white can take queen side space while undermining the defense of the pawn and generally wind up with a better position. In my experience in the Catalan system, black tends give the pawn back and accept that white had to waste tempi to do so.
Im not a Catalan player myself, but in general, after giving up a pawn, it is not always the right question to ask whether you can recover it. The Queens Gambit is ofter referred to as a pseudo-gambit because recovery of the pawn is often very simple. In those cases (say the Muzio Gambit) where recovery of the material is by no means assured, the right question is how to make use of the compensation. The Catalan seems to me to be an in-between case. White can play to recover the Pawn but doesn't have to. Black can play to hold the Pawn but doesn't have to. For more detail, you need advice from an actual Catalan player.