Typically you'd install the tub (plumbing and all) first before drywall. Alot of tubs/showers are attached to the studs, then you drywall down to the top of the tub and over the mounts.
Tidbit: On fiberglas tubs, I usually mix up a bag of quickrete and set the tub down into it. Makes it much more secure and as of yet (oldest one is pushing 10 years old) no problems.
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