Excellent writing considering you were on a phone - must be an iPhone
So let's step back for a moment... under the silver-painted plastic top cover, you presumably found a black plastic cam cover (the one with the spark plug coils mounted on it). Earlier engines had an aluminium cam cover as Stibbs mentioned, but I think yours is black plastic.
When you said "the crease of the head gasket", I think you meant that oil is collecting in the join between the cylinder head and the block. That's no surprise - oil runs down and along and sits in any gap it can find. As Smaky said, we need to assume that the head gasket is NOT leaking, because it is a lot more likely to be the cam cover gasket leaking, and if it were the head gasket, you would probably have oil getting into the coolant (expansion tank filling with brown gunge).
The cam cover gasket is actually a complex-shaped rubber seal that includes rectangular seals for the spark plug holes (earlier engines had these as a separate part). The rubber hardens and cracks with age, so can't be reused. Because it is a complex shape, it may cost a bit to replace but it is worth it (you can't cut one out of a cornflake packet, cork sheet, etc. as some non-Alfa people (old English car enthusiasts) may recall in misty-eyed nostalgia.
So, from the point you were at, you next need to take off the spark plug coil bracket. Undo the four Ribe bolts that hold the bracket in place - no need to take off the Torx bolts that hold the coils themselves, because the whole bracket complete with coils can be pulled/prised upwards off the spark plugs. Note that one bolt has an earth wire attached to it (remember that on reassembly).
Then you can undo the Ribe bolts that hold the plastic cam cover onto the cylinder head. There are bolts in each corner and along the sides, and also a bolt halfway between the two camshafts.
To remove the cam cover, you'll have to unclip and remove the wiring first.
Removing the old gasket is easy provided someone hasn't used sealant liberally (if they have, it probably didn't work because of the oily, hardened rubber, and you have to clean all the sealant off to do the job properly).
However, when you fit the new gasket, put a little sealant on the corners of the gasket at the cambelt end. That helps it to seal against the join where the aluminium cam bearing cap (behind the pulleys) meets the cylinder head. I believe this sealant was originally applied at the factory and shows as small 'ribs' on the aluminium (which should be cleaned off, but it shows you where to put the sealant).
I expect that the person who replaced the cambelt didn't have a new cam cover gasket to fit, so they put the cover back on with the old gasket and it has slowly leaked ever since.
By the way, the terms 'cam cover' and 'rocker cover' are interchangeable - I just prefer 'cam cover' because there aren't actually any rockers in this engine
The cams operate directly on hydraulic tappets. No need to touch any of those of course, you only need to fit a new gasket and I think this problem will be solved.