Here's what Merriam-Webster Online has to say:

Main Entry: gig
Function: intransitive verb
Inflected Form(s): gigged; gig·ging
Date: 1939
: to work as a musician

8 different meanings for the word gig...Had to go to number 7 to find the one that deals with music.

I like the "pronged spear for catching fish" one myself.

mike