Now we gon' break this thing down in just a few seconds.
A
These spawn exits are bad for both RED and BLU. For red, they force the defenders to divide their attention between both fronts, turn one way to try and deal with the Soldier/Medic combo spamming at you, a Sniper and Scout blindside you, it forces and encourages RED to stay inside the building of A and that small area on the back of A. And this makes it difficult for BLU as well. If all of the RED team, Sentries, Heavies, Demomen, you name it, are cooped up in little building, it's going to take a monumental effort to shift them out of it.
Furthermore, in Gravelpit, BLU's spawn tunnel exits never face the same place, or deposit attackers in the same place, which gives them more leeway and options for capturing the point. Not the case in your map, yours face the same way and lead to the same place, essentially the same thing as only having one exit, which would understandably restrict BLU's choices in trying to attack A. Right now, to capture A in your map, the best bet is to capture B and loop around via C to cut off reinforcements and try and wrestle RED out of A from behind, which I'm sure you can agree is not good.
What I would propose to help alleviate A's problems is twofold. Firstly, move one of your spawn tunnel exits to the low ground. This means BLU have a more out of the way spawn exit that helps them flank and RED don't have to worry as much about which door they need to watch.
Secondly, let people stand on the roof of A and make a hole in the roof, so that if RED do decide to do as they did in today's test, and pile multiple Sentries inside A, BLU have a good angle to counter them. It'd probably be best to nobuild the roof though.
B
I think Fr0z3n put it best, and I will attempt to paraphrase him, "I'm looking at all the places people can shoot me from, and there is a lot of them." As a defender on RED, there's at least half a dozen places damage can come from, most of it from far away or from a tiny window or from a tiny window that's far away far away. Simplify. One large opening in a wall is better than a lot of small ones. It'll streamline navigation as there are less doorways to traverse through and give RED a few less windows to watch.
Now even if you do make it so that RED don't need to worry about being attacked from all angles, B would still be pretty difficult to defend, I imagine, on account of not really having a good place for Engineers to nest up, I suggest expanding over to the left side, (RED's perspective) and giving them some kind of structure there that they can defend from rather than sitting out in the open.
C
Goddamn I have been typing this and making pictures and procrastinating on it for literally a few hours now, let's make this quick.
Fr0z3n is right once again, and C has the issue that attackers don't have many any good way to see the point which forces them to go in blind. In addition to this, the routes BLU have into C put them at a disadvantage. All of the routes BLU can take into C exit underneath some higher ground for red or directly into a massive Sniper sightline. I recommend two changes.
The first being simply move the exits to the routes so that they aren't left at such a heavy disadvantage.
And rejiggering RED's spawn so that Sniper can't just stand outside of their spawn door, snipe a guy, and run to a resupply locker two feet away when they get spammed. The below pictures would involve removing the upper level to the spawn entirely.
That took a long time, I hope you found at least some parts of it helpful and didn't think it was just me pulling ideas out of my arse. Talk to me on Steam if you want me to clarify something, I appreciate the quality of my explanations may have diminished as the post went on, just talk to me once I've had something to drink.