Gas MIG welders use a shielding gas to protect the weld pool from the surrounding atmosphere. The advantages of using gas MIG welders are that they provide clean, splatter-free welds with no slag. However, they could be better for outdoor use, where wind can blow away the shielding gas.
On the other hand, gasless MIG welders use a flux-cored wire, eliminating the need for a separate shielding gas. These machines are more portable and best suited for outdoor use. However, flux-cored welding produces more spatter and slag that has to be cleaned up. Understanding your specific needs can help you decide between these two types.