If you want to remove the camshaft cover yourself for inspection then the gasket should be re-usable.
It really isn't worth just changing the timing chain tensioner on its own as it is part of the kit anyway. Your better off having a new timing chain kit fitted and it's prudent to have a new balance chain kit done at the same time (as mentioned above).
I would also recommend fitting genuine GM (or FAI) kits if your budget allows - many garages will fit third party kits, e.g Lucas and they are a struggle to fit.
