Is it ever a good idea to try to hold onto a pawn in the Queens Gambit?

So

I played this game recently, and my question is whether or not you can safely hold onto the pawn in the QG.

Sometimes it is, and sometimes it isn't.  It depends on the details of the position.

There are variations in the slav and the semi-slav where you can hold onto the pawn, in the queen's gambit accepted it is not possible, white can get back that pawn when he wants to.

Incorrect.  In the queen's gambit accepted it is NEVER a good idea to try and hold onto the pawn.  There are variations of the queen's gambit declined where the pawn is captured later and black can hold onto it as csalami10 pointed out, but I don't think that is what you were referring to.

Agreed, although white sometimes plays it wrong. But it is not something ou should aim for.

In the QGA it isn't a good idea.

Rarely, but not "never."

Yes, Semi-Slav (classified as its own opening but stems from the basic QG position)

Of course you can't really say it is "never" a good idea. There are always exceptions, for example if white really plays quietly there are surely some lines black can successfully hold on to the pawn. But it is not usually possible.

Well, a Semi-Slav isn't a QGA.

But in, for example, certain of the 3.Nc3 lines, white may choose to play the line as a true gambit, focusing his play in the center and on the queenside.  And if he does, black's best opportunity to wrest advantage may well be to consolidate and seek counterplay on the queenside.

1.d4 d5 2.c4 dxc4 3.Nc3 Nf6 4.Nf3 a6 5.e4 b5...

is a line that gets seen at the GM level.

probably bad especially you play a good strong opp.

Not a bad opening? It's considered one of the most logical responses to 1...d5 and gets played at all levels. You don't need too much theory knowledge to play at at amateur level and can get by knowing the basic ideas.

Be nice? I didn't mean to be rude and I never 'put words in your mouth'. Not a bad opening is an understatement, that's all. Demeaning is relative, if you said the wing gambit was not a bad opening, that'd be considered quite good. Again, none of this is meant to be the least bit rude, you don't have to call me a noob, that just hurts my feelings. Sticks and stones may break my bones but words leave psychological damage that will never heal.

See? That there is demeaning and rude. I'll stick to blathering.

In Searching for Bobby Fischer (when his opponent playing white played Queen's Gambit), didn't Joshua CAPTURE the c-pawn ?  (Camera angle is unclear.)  If so, does it mean, "I'm up for a fight!"?

Alright grandpa, have it your way.

You did not communicate well.  So it's probably best to stop with telling others they deserve what they get for dealing with your shortcomings.

Black has a decent game after 3...b5 4.a4 c6 5.b3 e5, but the real problem is 5.ab5 cb5 6.Nc3, when he is effectively forced to play the clumsy 6...Bd7, when white has strong compensation with both 7.Nf3 and Berliner's 7.d5!?