How to cleanly remove, and reuse, an area of plasterboard (drywall)
It's not practical or cost efficient (even with &quot;free&quot; labor) to remove drywall for reuse.
However, since what you are (evidently) &quot;really&quot; after is an access panel to your piping, you might get away with the following:

Identify via strong magnet locating lines of fasteners, or your
stud-finding method of choice where the studs are.
Identify as closely as possible by measurement where your pipes of interest are.
Cut from stud to stud somewhat above and below the area of interest, to provide adequate working room. Cut along side the studs to release that section of plasterboard, and remove it.
Do whatever needs doing in there.
Realize that plumbing access panels are a Good Thing® and frame the opening with some simple wood strips
Install a wooden panel slightly larger than the hole or use the cut-out plasterboard as a panel screwed to your strips
If using plasterboard, install some thin wood strips to cover the joint (do not apply joint compound - the whole point is to not have to rip the wall open again the next time, because there will be a next time, if not for you then for the next owner.)