IF this tub is also going to serve as a shower, don't use drywall - use cement board (cbu). Assuming it is going to be tiled, don't run the cbu over the flange or it will bow out making it tough to make the tile look good - depending on the size of the tile, as long as they are big enough so most of it is supported by the cbu, you can stop the cbu at the top of the flange. If you wish, or if the tile is small, then either notch the studs so the thickness of the flange is even with the stud above the flange, or shim the studs so you can run the cbu down to about 1/4" from the tub surface. Also, put up a vapor barrier behind the cbu and run it down over the flange to protect the studs from moisture damage. If you are going to tile, check out
www.johnbridge.com.