The surface of the mating face of the gasket (where it seals to the head) has a couple of very slight raise ridges along the length of it to create a better seal as they squash down.
I would never use silicon sealant on any gasket (or in place of one). It can cause untold damage to the engine if any of the excess "squeeze" breaks off and manages to get stuck in any of the oil ways.
The most common leak from the cam cover gasket is at the front left (looking into the engine bay) because the gasket has to seal into a sharp 90 degree corner and it isn't easy to achieve a 100% seal using flexible rubber gasket. Plus the engine is lying in a forward position so gravity means that oil will always sits in a pool along the front of the cam cover gasket where it will easily find it's way through any part which isn't sealing 100%. A bit of liquid additional gasket (used sparingly) might help the rubber gasket to achieve a better seal in that corner but I certainly wouldn't use silicon sealant.
Yes I'd also recommend buying a new gasket.
At least get the cam cover off and have a check on it - if the rubber has hardened into plastic then it wont seal. It should be very soft and flexible.
If you are stuck and decide to use sealant at least get the non hardening type - less chance of it getting stuck anywhere in the oilways as Giallo says.