Ok, technical reasons;
As RS4s mentioned, the main reason this gasket will leak, is because the shrink & harden; due to the heat radiating from the exhaust manifold & turbo. Repetitive heating & cooling does damage rubber seals in this way.
This particular gasket is likely to be better than 10 years old.
Once the gasket has shrunk & hardened , no matter how tight you do up the screws the gasket doesn't push down hard enough to seal properly, nor will it conform properly to the jointing surface; because the rocker cover has bottomed out on the head.
Repetitive heat cycling is also the reason I wouldn't use silicone to reseal the gasket.
Very few oil jointing silicone gasket sealants are rated for more than 200-300deg. The exhaust manifold & turbo can exceed 700-800deg.
I'd be willing to bet the one on the intake side is not leaking at all.
Using silicone to fix a leaking gasket is false economy. When a gasket leaks, you replace the gasket.
The leaking gasket is the ROOT CAUSE; silicone is just a band aid solution.
The only other possibility at this point is that the screws that hold the rocker cover down, may have come loose. In this case I would still recommend replacing the gasket due to it's probable age.
When a customer pays me to do a job; they do not expect to have to bring it back again because I tried to save $30-$40. Also, my profit is gone if I have to do it again; along with my reputation.
Regarding the loose nut, as above; use a griplock nut or "hugnut" otherwise you could use a lock tab under the nut. Locking compounds aren't really suitable here as most loctites are only rated to 300-400deg, then they break down.
Do the job ONCE; do it RIGHT.
Hope this helps, Dale.