If I can make the assumption that this is preboil or on the boil kettle, then the weld process is less consequential then if it were on the cold side of the brew house.
Yes, there is a proper, proven way to do things. But as with all things except space travel and nuclear or biological experiments, there is wiggle room to achieve acceptable outcomes. While this may not be the bedt application for GMAW, it can still yield a decent result. The best chance of success is to purge the root side of the weld joint. This regardless of process.