Installing new soaket tub

Hi, Watching new house going up, they install tub and drywall goes on(the green water resistant one)

Only in lowest-end houses. Drywall has no business being that close to water, even the water resistant kind.

The green drywall was a mistake they came up with after mud-bed tile jobs became too expensive during the building boom of the 1950s and 60s. (Or they didn't have enough skilled tile-setters to do them.) It is no longer considered good practice, since it is only water resistant, not water proof. Grout always leaks, eventually, as does the water seal where faucet handle penetrates the wall. Most regulars on this group can describe all the mushy green drywall they have ripped from tub enclosures over the years. Use cement backer board on the walls around the tub, and use the green drywall (or better yet, the yellow non-paper stuff), in the rest of the bathroom where it only gets damp, not wet.

And yes, tub goes in first, against the bare studs. Use this once-in-a-lifetime opportunity to insulate around and under the tub, especially if it butts up to an outside wall. Guys seldom take baths, so we usually don't care, but SWMBO will appreciate the bath water staying warmer longer.

Yep, and it's only done, today, as a low-end corner-cutting measure. Any DIYer doing it is simply nuts.

Quiets the tub, too. Cheap enough.

If you are putting a fiberglass tub put sand under it