It is necessary to glue the gasket to the valve cover, otherwise it will fall off when you turn it right side up to install it. Wait for the glue to set, otherwise the gasket will still fall off. You can use Three Bond 1184 or grey silicone.
Clean the glue remnants out of the groove in the valve cover before gluing the new gasket in. Rather than trying to put glue in the groove, it is cleaner to put a bead on the gasket. The way to do this is to install the gasket in the cover without the glue and lift it out in sections to glue it and drop it back in place as you go. Apply the glue only to the end of the vertical bar of the T-shape of the gasket's cross section. Also put a little dab in each of the two corners of the two half moons of the gasket after the glue has dried right before you turn the cover over to install it.
Downward force on the valve cover plays a role in whether or not the gasket seals. How much downward force is exerted on the valve cover by the valve cover bolts is controlled by the condition of the valve cover bolt grommets. The valve cover bolts have stops built into them, so tightening beyond the OEM specification of 7.2 ft.-lbs. will not contribute to better sealing, but brings with it the risk of breaking the bolt, stripping the hole, and/or breaking a cam cap (these have never been sold separately from a complete cylinder head, complicating repair). We therefore recommend replacing the valve cover bolt grommets any time the valve cover gasket is replaced.