A gas check does several things: it strengthens the outer edge of the base of a lead bullet where pressure, friction and stripping on the lands make it susceptible to lead on the barrel contact and it keeps layered leading down as it scrapes the bore at full diameter. This is why they can be driven faster at higher ambient temps in a hot gun or rough gun, repeatedly.
"Gas" doesn't melt a lead bullet base as any fired, recovered bullet will show. There's no melting around the sprue or base or unchecked shank at all, ever, EXCEPT that outer edge where pressure and friction and land engagement with the barrel contact is made.
The base of a bullet is the area that has the greatest pressure applied to it and is the weakest point of a lead bullet; if there is too much pressure (heat), friction (heat) and it strips (heat), it can lead.
