How do you Remove glue on a steel garage door?

It would have helped a lot if you specified what exactly, rather than just 'glue' - there are many different kinds with different solvents. However, try Goof Off, that will soften many glues. Rub with a rag dipped in this and it may soften enough to peel off. Other wise, scraping with a piece of hardwood shaped like chisel may get it off too.