After he'd auctioned of Runjhun's marriage items, instead of letting Abhi & Runjhun assume on their own that he was done with them, it felt like him telling Abhi that he was done with harassing Runjhun was as close to an apology as Guddu would probably ever get. Like he did realize that maybe it was a little too much done in his anger, and just wanted to let go of the matter.
Even as Runjhun scolded him and spoke her mind, he never said a single word except for when she started burning the things. Then he seemed hurt which quickly again changed to anger.
I'm not trying to defend Guddu in anyway, but it just seems like his character does seem to have an underlying current at times where his temper takes the better of him. After touching Runjhun he seemed to be goading, and was smirking that he'd succeeded in doing exactly what he intended to with Runjhun, but there was a moment when he lets Runjhun go that there is a brief look on his face that suggests remorse in his own actions. Where he doesn't even look toward Runjhun as she stumbles out.
These expressions are when you consider that there is much more to Guddu then what he shows in his outwardly character, of being selfish and doing what he pleases, and caring for no one else. Or maybe it's just me trying to find redeemable traits in Guddu's personality.