Trout, greenboard hasn't been to code for a few decades.

At the very least, you need backer board .... typically, fiber cement board, taped at the seams and all seams/corners, etc., painted with Red Guard, or equivalent.

What I just described, is considered "old school", although I remember when greenboard was used in bathroom situations. However, even thirty years ago that was NOT considered best practices.

Today, it's against code. All greenboard is ... mold resistant drywall. That's gypsum board. Take a sheet or scrap of drywall, better yet ... green board drywall out the next rain and see what happens.

So, if it's a DIY ... NOT best practices, but code compliant .... fiber cement board, taped seams, Red Guard painted over the thinset of the seams, etc.

Should get you by until you're an old fart like me.

Installing a tub is not that difficult. Back a few years ago when I got ultra bored and real estate prices were almost giveaway prices for cash buyers ...

Kinda added to my portfolio and did quite a few tubs, surrounds, bathroom renos. Nothing like expanding them, just replacing floors, vanities, tubs, etc. Still pick one up, hither and yon ... More of a place to store excess tools .... And an excuse to buy more!

But, don't put drywall around the tub.

I like to use the same tile for the surround that I did on the floor, or something that really contrasts. I'm just not into the look of wall tiles. Just remember, the larger the tile, the stronger the sub floor has to be. ANSI standards are 10^ joists (minimum), 12 inches on center, with 1 and 3/8 minimum plywood subfloor. If the joists are 16 on center .. I'll cheat and sister and brace ... really not supposedly acceptable, but I've encountered no problem. Then, I'll laminate two sheets of 3/4 ply (that's screwed and glued .... and remember the second sheet should NOT be screwed into the joists).

Sometimes, things sound like a great deal of work and aren't . Sometimes things sound easy and are NOT,

But, back to your tub surround .... do NOT use green board.

Shoot that worthless POS at Menard's. Save big money and get terrible advice.,

BTW: Wish we had a Menard's here .... they bring down the prices at Home Dopey's and Blowe's. Not only are they cheaper .... they bring down the prices at their competition.