How do you soften leather gardening gloves that have gotten wet and sat for years

Toss them in a drier on cool... or you can rub some mineral oil on them (not motor oil or vegetable oil.)

Neetsfoot oil. A light yellow oil obtained from the feet and shinbones of cattle, used chiefly to dress leather. Find it at a farm supply store. Unless these are very expensive gloves, you can buy new ones for only a couple of $$