this is the stuff. it comes in a toothpaste style tube, a pressurized can or the caulking gun style. just buy a package big enough for the work at hand because it will harden up quickly in the tube
https://www.permatex.com/products/ga...et-maker-5-oz/
I clean all the gasket surfaces with brake cleaner and dry them. also blow out all the bolt holes (do this fist) because if there is oil, old silicone or liquid on the hole this can cause a hydraulic situation under the bolt when it is installed and fluid can be pushed back out around the threads and cause an adhesion issue with the sealer. it can also cause issues with torquing and, worst case scenario, I have seen it crack castings. then install the sealer, doing a small circle around each bolt hole as you go, and lastly the cover. sometimes installing a stud in a couple of the bolt holes will help line up the parts and if doing, say, an oil pan while it is upside down there are installation helper studs that will hold the gasket in place and then will hold the cover in place while you check all the edges and corners etc and install a couple of bolts. when all the bolts are in and the parts are lightly mated then the studs can be removed. they work awesome even if the repair isn't upside down.