How do I fix a fiberglass tub with a gasket that doesn't seal very well on tub shoe?

Is the bottom of the tub rough? They may not have sanded or smoothed the mold marks. Can you access the underneath? The drain end doesn't thread in enough or not tight enough?

Take a tub drain wrench, and tighten the tub drain flange as far as it goes. If it turns 1/4 turn and more, that may be the fix. If it doesn't turn, then unscrew the tub drain flange, remove it, push the old gasket to the side until it falls into floor cavity, carefully insert new gasket, and reinstall existing tub drain flange. All this work is done from inside of the tub. No need to cut ceiling.

Let us know it went

Milo