Go to an auto parts store or body shop supply store. They will carry an adhesive remover that is used to take off glue and residue from decals, bumper stickers, tape etc on cars. As long as it is on metal this will work fine.
Keep it away from any wood. It will remove the finish.
A cheaper alternative to try first is lighter fluid. use cloth such as an old t-shirt. dampen a small portion of the cloth and rub the glue gently at first. Then use another section of the cloth with some more lighter fluid and wipe away the glue. Repeat as necessary.