Do you fully understand Special Relativity?
I'm learning GR from Hartle's book at the moment. I'd say the biggest prerequisite is to have SR nailed. Also, definitely Lagrangian mechanics. Hartle covers both of these, but really only at a revision level.
Obviously knowing classical gravitation helps as well.
I've reached Chapter 9 (Schwarzschild Geometry) and I'd say I haven't needed Topology or Differential Geometry yet. The Differential Geometry has really been what I would call vector calculus.
Hartle, it seems to me, is quite accessible. I'm judging from experience of it alone, but it certainly isn't a mathematical minefield. The only issue is that he gives few hints and no solutions to his problems. But, then, that's where Physics Forums can help!