What does "work on side gigs" mean?

Solution 1:

A gig in this context just means "a bit of work"

2 - MAINLY AMERICAN a piece of work that you do for money, especially if you are self-employed.

(Macmillan Dictionary)

And so a "side gig" is a bit of work "on the side", meaning "outside your normal employment".

So, it's saying that some of the people were in the Yahoo office, ostensibly working for Yahoo but in fact doing paid work for other clients.