the grommet slide over the casting 1 lip inside
the other outside so is locked in place.
the plugs need to be off to slide the wires thru.
unless a slot is cut into it.
Having said all that.
I have used a product call "Right Stuff" to seal grommets
its high pressure in a can sticky as $%%^
silicone isn't even in the same "Ball Park" as far as
stopping leaks and replacing/coating gaskets
a skim coat on gasket = on leaks.