The gypsum board (drywall) will be fine. It helps holds houses up, so it should hold your tank up.
Although don't be fooled, the end that I can see in the photo is OSB (chip board), not plywood. Plywood stands up to wetness much better. How do they plan to tie in the OSB end with the gypsum board? Trying to mud over it won't work...the OSB will expand and contract like any other wood, and the gypsum won't. The corners, and any joints will crack.