Do grandmaster's blunder?

Yes, they absolutely do.
I remember seeing a video of Magnus Carlsen making a blunder and realizing it too late; when his opponent appropriately punished it, he just sank into the chair 💀

Well maybe these two particular players blunder sometimes but just not in this game

Grandmasters blunder less but they still do but not a lot
Yes, As grandmasters are more experienced in the game t hey don't blunder as often

Yes, they absolutely do.
I remember seeing a video of Magnus Carlsen making a blunder and realizing it too late; when his opponent appropriately punished it, he just sank into the chair 💀
Was that the video where he got mated in 9 moves by castling?
Take a look at this game between this grandmaster and this non grandmaster:
https://www.chess.com/analysis/game/live/123319205351?tab=review&move=0
Surprisingly there were no blunders