Offensive GG is on the ruder side, yes. It's good manners to wait for your opponent to GG first.
My personal preference is to ignore them and still send my zergs into their crystal lines.
Twice.
One guy said that and then proceeded to try to lure me into stillness. He ran into my constantly building hydralisks, I guess hoping to 'bait' me into assuming he'd left.
He did shortly after.
I was just playing a game on difficulty 8 with Hard Golumns turned on. I had about 250 when the AI said "GG" and including a Mark II Hunter/Killer in the Exo-wave.
On behalf of the AI War neophytes (which I include myself), I got most of the first message. I assume the "250" refers to the AI Progress. Not sure what the 'AI said "GG"' refers to.
Human Translation:
I was playing a game at over normal difficulty with Hard Golems. At ~15 planets worth of capture the AI whomped on me by including the unit Hunter/Killer II in an exo-wave I was utterly unprepared for in the resultant exo-waves. Are these common?
Answer: Yes, after a certain point. If you can't handle H/K's you never let your AIP over ~170. To handle H/Ks, you need a chokepoint or two and a huge stack of forcefields.