Sorry this is the new one that is plastic, not the previous picture of the old. My fault for not being more specific.Silicon adhesive is definitely not a good way to stop a leak. Thread seal tape could work if the the parts are metal. They look like metal so that’s probably the way to go. It works on pvc too just not really recommended because you just need to make sure you don’t overtighten since seal tape makes tightening almost too easy.
Between the gasket and the bottom of the basin. I can tighten the jamb nut but there's still a small trickle coming out at the top of the gasket. It is like it didn't seat in the hole at the bottom despite my best efforts!
(picture for my own benefit)