One of the most popular is fiverrdotcom. Aldo there now are several fiverr clones...just google " sites similar to fiverr". Hope thus helps
There are are several free advertising sites that you can.That's a good place to start, it's free all it takes is a very catch and interesting contents
Yes, nobody calls their services a "gig" unless they do business on fiverr - so when someone comes on here talking about gigs you can be 100% certain that they are wanting to drive traffic to their fiverr account.
The only place to sell a "Gig" is fiverr, if you want to sell your "services" try classifieds, craigslist, SEOMarts, and how about the good old word of mouth in a town near you? What is it you're trying to sell exactly?
The niche in which you are will determine how you will promote your gigs. If you can specify the niche, then you can receive answers peculiar to that niche rather than the general means of promotion which may not be applicable to what you are selling.