In gasless MIG welding, a flux core wire is used instead of a tank of inert gas to shield the weld.  The flux melts and provides the gas needed to shield the weld. Gasless is not as smooth, and it produces slag, which needs to be cleaned from the surface of the weld. It’s harder to achieve a clean and defect-free weld when you’re using flux-cored wire. They tend to spatter much more than using solid wire and gas MIG welds.