Can I use silicone to glue drain gasket to drain before setting tub?
A thin layer of silicone won't hurt but make sure it is evenly applied in case it takes longer to place the tub and pull the drain tight. Several houses back I was doing a remodel that the original owner used a thick layer and let it dry prior to installing the tub I don't know how many years before but it had a small leak and I had to replace two of the joists and some decking because of the leak, but a thin coating will help seal and hold it in place while setting the tub.