While yes, the gas will go around the panel, but it won't fill the empty spot where the MG gunner is sitting on, because SS13 code ain't that complex.
I will try to find video evidence, but I assure you that protecting an MG gunner from gas with the MG emplacement itself and 2 glass panels hugging him works. It was thought up by Surr, and he also spent a couple of rounds proving it as a real thing.
If there've been changes to this whole interaction, then I'll be proven wrong, but I haven't seen anything about it as of now
Red = The gunner location
Blue = The glass panels
Black arrow = MG and the direction its facing.