@GzUp is correct. If GGG did not have good defense, someone like even a Curtis Stevens or David Lemieux would've landed much more and at very minimum rocked him. He does have good defense, because he slips tons of punches....which he also did vs Jacobs as well. GGG also knows how to create space to reset his offense if need be. Basically, he's aggressive offense + his ability to slip punches while coming inside and stalking you = good defense. It's sort of Mike Tyson'ish/Joe Frazier'ish like how he slips punches on the way in to land a bomb on his own. If he did not have defense, he'd be tagged over and over and over, blocking punches with his face while coming in.....and that's not Golovkin at all.
GGG has good defense for someone who employs the style he does. He's not a Provodnikov or Matthysse or Maidana at all.
Compared to them yes he has but compared to them who hasn't? I wouldn't say he has a bad defense for his style but he isn't a JCC Sr. or Duran either.
Of course he needed some defense to even become a top fighter. I just don't think his defense is good enough if you compare it to other guys around his level.
The only time I was impressed by his defense was against Lemieux
on other fights there were two kind of scenarios
A: His offense functioned as his defense and his opponents didn't dare to throw they just wanted to survive.
B: When he faced more skilled opponents, they hit him without much difficulty.
He used to have better defense btw but he got older, maybe he started to believe in his power too much and he also stepped up (or got the opportunity to step up... depends on how you view it) in competition finally with the Jacobs fight.
On his last two outings and also against Monroe or Murray, Golovkin's defense didn't look that good. He was there to be hit against fighters with slower hands than Canelo he will also there to be hit against Canelo.
I don't think Canelo has an A1 defense either but better than Golovkin's. Even when he invites opponents to throw on the ropes he doesn't seem to eating blow after blow. Of course he should not do that shyt against Golovkin though... But he doesn't always do it, for example he didn't do against Cotto, I think Canelo knows what's up...