I saw a gargoyle on a light that hangs close to a business and not a Catholic church. Why is it there?

It is a misconception that only Catholic Churches have gargoyles. Non religious buildings in New York City also have gargoyles. They are often used as water spouts (rain gutters), but often they are just for ornamental purposes.

It would seem that somebody wanted to place a gargoyle there for reasons unknown to us.

If the light is affixed to the side of a building, evidently the building owner at the time the light was placed there felt like adding a gargoyle. If it's a public street lamp, some town or city employee made that decision. Maybe that person figured that a gargoyle would make that street a tad more interesting. After all, not too many street lamps have a gargoyle affixed to them.

Decoration. Ever heard of that?