Aerostich makes waterproof fabric lobster-claw glove covers which may be the best solution, especially in really cold weather, and you can wear any glove you want underneath. Most "waterproof" gloves (even leather ones) use a Gore-Tex (or similar) membrane. Just Google motorcycle waterproof glove reviews. Sno-Proof makes a really great waterproofing wax for leather boots, but it makes leather gloves feel yukky. I tried it and had to throw away the gloves.
"I tried some camping spray that is used for tents and other camping gear and it didn't work."
Those sprays contain chemicals that may well damage leather. I can't imagine why you would use it...
"...is there a specific chemical/product for leather ?"
Several. Nikwax, Renapur, Collonil, G-Wax, Dubbin, Gliptone, Tarrago and Ecco all spring to mind.
Most leather riding gloves don't work well in the wet unless they are made specifically for wet riding.
Buy a second pair of gloves for wet riding, or get some overgloves, like Aerostich 3 finger gloves. Another solution is to wear latex gloves - like your doctor wears - under your normal riding gloves. They will keep your hands dry and help keep them warm.
There's lots of different oil based projects that will work, simply because the oil and water do not mix. However you need to realize that this doesn't make them waterproof, rather water repellant. There are certain waxes that will work as will.
I would consider Red Wing Boot Oil
Easier just to buy waterproof gloves.
I would suggest Always Dry by Nanex
theres wax's for that kind of thing, not sure of the brands sorry.
