Pros: It's gravelpit.... that's a pro for some people.
Cons: You've made this map way too small. I'm not sure why you choose A as the focus for gravelpit. I would have choosen C.
Improvements: I think you should scrap this idea. Honestly, it's too small. Spawns are way too close to each other and there's nothing between the two of them. It will easily become a death fest right out of spawn.
I'd move the map to focus on C and put Red spawning from the entrance coming from A. Remember, in arena maps, the spawns remain open. So really you could keep those tunnels and a good portion of the area going to B (the shacks, and upper area - not the point).
Notes: I won't be adding this version of the map to my server. Unfortunately, it's too deathmatchish and not enough arenaish. I hope you take my review seriously because I'd still love to see an arena gravelpit.

Good idea if the spawns are in the right place.

You should do 3 diff versions for each zone.

I'd like to know how this would be fair for both sides...

Arena matches are supposed to have equal side bases, the same with Ctf maps, where the bases are pretty much identical, but for this, it's completely different. I don't know how well this would turn out, but I would like to assume that Red has the upper hand in an arena match on this map.

Pros: You didn't do a crappy conversion and everything looks normal.
Cons: Why did you take everybody's least favorite stage instead of C or maybe even B?
Improvements: Not much to improve considering Valve made 95% of the map.
Notes: I'd really like to see an arena version of C.