Is is too late?
No, I don't think so
What should I consider?
Does the door still operate or is it merely decorative?
i.e. will repairs to the mechanism be necessary? You may need to grease, repair or replace the wheels up top and/or fix up the rail it hangs from.
Or are you happy with it stuck open and you just want it to look nicer different?
There is ...
As pointed out in the comment, you can of course do it with a basic triangle calculator.
Do yourself a huge favor and buy a speed square. They are $10 and are useful for so many things beyond just marking an angle on a board.
I will line up all the posts, measure, mark, pre-drill, and screw in all the brackets before the posts ever go into the ground.
What if you can't drive one all the way to your predicted depth, or what if you drive one your predicted depth and it is not yet stable and you want to drive it further? I would not pre-attach your rail brackets or I bet they will ...
Yep. It can fit in your pocket.
The whole point of a bevel gauge is to transfer angles as simply and compactly as possible. It's a carpenter's tool. You wouldn't want your utility knife to dice vegetables for you, and you don't want a fussy, clumsy tool in your belt pouch that you don't need.
knowing that there's a floor above adds confidence that the beams above the ceiling are strong enough.
but you're going to need to determine how your ceiling is contructed.
Either the drywall is attached directly to the floor joists in which case a stud-finder will help you determine where to screw or the drywall is attached to battens which are run ...
If you have access to the studs, rather than fastening through a wall covering, you can assure strength by sistering to the studs.
If you use bolts instead of screws, you can make use of washers and nuts to add tensile strength where it is most needed. If you have access to the top side, you're all set. If not, rip the added segment to be narrower, you'll ...
The chances are you are not unwarping the board. You can try but its been there a while and will probably go back. I would hit the really high point with a planer, get it close to even then sand (for a while) this thing flat. Yes it will take some labor time but the alternative of trying to make this straight will take far longer and probably not work.