Regular G&G metal lines (with a few exception) don't have a dust cover release, but can be fitted with one. Their plastica lines have the dust cover release. No idea why they'd put the good feature in the cheap guns and not the metal guns.
I know first hand that VA's G&G have working releases, won't surprise me if other cansoft G&Gs do as well.
"The Bird of Hermes is My Name, Eating My Wings to Make Me Tame."