I'd like to know whether the word gig (temporary job) is formal or colloquial.
Always that I've read it, it has been in forums. I have never heard it or read it somewhere else, just in forums, which make me wonder about the formality of this word.
Colloquial when referring to any job, usually just used for casual employment, e.g. "I got a gig as a gardener for a few weeks".
But if you're a musician, I'd think you could use it formally and it would make sense.
GIG noun: : a job usually for a specified time; especially : an entertainer's engagement.
Source: Merriam-Webster Dictionary
gig (n.2) "job," first used by jazz musicians, attested from 1915 but said to have been in use c.1905; of uncertain origin. As a verb, by 1939. Related: Gigged; gigging. Source: Etymology online
Given the origin and use I'd say it is used more on a colloquial basis.