When installing my old rocker gaskets on my Buick/Ecotec/L67 V6, these gasket keep falling/slipping out when I try to put the cover back on. What can I do?

In order to prevent this from happening you can assemble the rocker cover and gasket on the bench with some silicone sealant, allowing it to set. Once the sealant has set, you'll then be able to install them back on the cylinder heads without fear of the gasket falling/slipping.