It is a slang word for "job" often it refers to music bit it can also mean other jobs: I got a gig playing guitar in restaraunt. That's a good gig ( meaning someone got a good job with good pay or some other advantage)

I guess it might have diferent meanings depending on the context, but is "concert" a good equivalent for you?

Many musicians don't have steady jobs, but get booked for single performances here and there. Musicians call these jobs "gigs." It's pronounced with a hard "g", just like "gigahertz" and not like "jig." "Last weekend my band had a gig at Club 42 and next week we have one at a wedding reception." The usage has spread from the world of music to mean any short, single engagement.