So you are a beginner, already using Libgdx, need collision handling and gravity, then you definitely should use Box2D. Learning it will be much faster (even if does not seem so now) and the results most likely much better (no offense) than breeding your own stuff.
Actually, in my humble opinion, I don't think box2D would make it easier for a beginner. I've personally attempted to use box2D solely for collision, but it didn't turn out to be easier than if I wrote my own collision detection. If you're using circles or complex polygons, or if you want to simulate real physics, then definitely go with box2D, but it certainly does come with a lot of learning about the api and such. If you're only using rectangles for collision and don't need to simulate real physics, then you're better of writing your own collision detection. It's not too hard to do, and it doesn't come with the amount of content and possible bugs that will come when using box2D.