Gcc was pretty wild-n-wooly back in the '90s, and seemed to add language extensions left and right, but that attitude changed quite a while ago, and it's very good about standards conformance these days (part of the reason for this, of course is it's no longer so necessary: many of these extensions have picked up by the standard in one form or another).

In my experience, gcc is also significantly better with standards conformance than VC++, either with default settings or with strict warning/conformance options turned on. [I was on a team doing shared g++/vc++ develeopment, and it fell to me to fix all the code checked in by devs using non-standard VC++ extensions... way, way, way too much time... >< ]