I've got my own opinions on GM plastic cam covers and 'experience' tells me the gaskets make no difference at all - that is cheap versus genuine. Torque them down at 6lb/ft as recommended, but age and engine mileage will always hold sway..... in other words if the engine has reached 6 figure mileage, then no matter what gaskets you use (or how clean your breather system is), oil leaks will be inevitable.
Recent events resulted in brand new GM plastic covers at £300 for the pair (which come with gaskets). A couple of months later I still run my finger along the seals and it comes out clean and oil free.
So in summary (and shoot me down in flames if you wish) plastic cam covers have a limited life span. Alloy covers may well last the course. Keep the breather system clean, and that will be the least of your worries, but rest assured, plastic covers do warp after a long period of time and are subject to hairline cracks, making the gaskets the least of your worries.... correct me if I'm wrong, just speaking from 20 years of experience with the X25XE engines.