Why is this transformer in my garage?
Since the transformer says "Gaslight Conversions", that's a pretty strong clue that it's part of a lighting system from Gaslight Conversions, located in St Paul MN, which also matches what's written on the transformer.

If you've ruled out all of your exterior lighting, it may be leftover from a low voltage lighting system that was replaced but never fully removed.

It's also possible that the transformer has been repurposed for something else by a former homeowner, but most likely it is (or at least was) used for lighting. -- especially since that company sells their transformer for $40, but you can pick up a generic 24VAC transformer from Home Depot for $10 - $15, so why buy the name brand if not for use with their lighting products? 

If it plugs in (as opposed to being hardwired), that makes it less likely that it powers any permanently installed equipment like a doorbell or furnace.