some early GL5 rated oils would become corrosive to 'yellow' metals (copper/brass) when the oil was above 250 degrees. If you hit that temperature inside a snowmobile chain case you have other things to worry about! Also, only the early GL5's had that problem, most newer ones also carry an API MT-1 rating, which is certified not to be corrosive. (an oil must pass ASTM D-130 to be MT-1 certified, of which one requirement is that it isn't corrosive to yellow metals)
But back to the original problem, as far as I know most snowmobile chain cases don't have any copper/brass anyway, but if they did, the Castrol GL5 stuff you bought IS certified as MT-1 also, so it is 100% non-reactive to copper.