I haven't looked around to see if there are ways to revert the cosmetic changes you dislike. Yeah the Radio Direction Finder (Hoop thingy) can obstruct your view, which can be a pill since you aren't able to walk around on the bridge to get it out of your way. If you give it some time you'll get used to it. When I first started using GWX it bugged me a little but now I don't even notice it.
Things like the bearing indicator in the binocs were removed for realism's sake. Again, something you'll get used to as you play. Especially if your knowledge of relative bearings improves. You'll start to just know off of the top of your head that a contact at bearing 040 is going to be off of your starboard bow. These are things that even if you're not a hard core "Full Switch 100% Difficulty" guy will improve your efficiency as a sub captain. Also don't forget that you can use the UZO for bearings as well.
The deck gun issue..
Again, a realism thing. In stock SH3 it's waaaaaay too easy to hit things with the deck gun, and it's annoying that you can't man the darn thing unless the seas are practically flat. GWX does make gunnery more difficult but it also allows you to man the gun in slightly less than ideal sea states- if you wish. As someone who has fired weapons off of a pitching, rolling small vessel it's not as easy as some might think- especially if you aren't using gyro stabilized equipment. The key to sinking vessels in a timely manner with the deck gun in GWX is to try to strike the waterline as much as you can to create flooding. You don't have to hammer away until you get the "She's going down!" message. Some well placed shots will cause enough flooding to where she'll go down eventually.
Again, it's something that with practice you'll get used to and get pretty good with after awhile. You also have to keep in mind that sometimes spending a torpedo really is the best way to go about sinking your quarry. Remember, that after a certain point in the war deck guns started to get removed from the U-boats altogether because they became a useless piece of hydrodynamic inefficiency.
I'm a little pressed for time, so I can't elaborate further but I'm sure that others will be along to help get you sorted out and on the way to working around the things that have come as a bit of a shock to you.