[Mart]

I'm going to be blunt here. I think you may be wasting your time. De-rusting something like a gearbox casing or old crescent wrench may be worthwhile but when it comes to precision engineered parts like gears and synchros you must surely be left with pits where the rust has eaten in.
Not trying to be a killjoy, but realistic about the potential outcome.
I hope you prove my conjecture wrong and have a good usable set of gears, but I don't believe that outcome is guaranteed.
Mart.