Sir Gaf is the pronouncement of CIRGAF, which stands for "'Cuz I Really Give A Fuck."
CIRGAF is used in a sarcastic situation when one does not care about what the other person is speaking of. This can vary depending on the person, from dramas, to quick chit-chats, to "the talk."
Sir Gaf is used in a rather introducing way or questioning manner, but still mean the exact same as CIRGAF, as well as used in the same situation.
You can see it used in the examples below, somewhat similar to CIRGAF, but rather in a dick way.
"Hey man, guess what happened last night, I met this bastard, we got in a beef right? So I was thinking of ..."
"Wait, is his name Sir Gaf?"
"...So how was YOUR weekend."
"Son, lets have our talk about the birds and the bees. This is very important to our family. I just wanted to..."
"Hold on dad, I think Sir Gaf is calling me."
"Hey, I didn't want to talk about this either."
"Hey Sir Gaf? How are you?"
"Okay okay, just use a condom."