There's a small, maybe 1/2 inch, gap between my concrete backerboard underlayment and the bottom of my drywall. I want to install Ditra waterproofing (mat plus Kerdi seal tape). For waterproofing the outside perimeter, I'm supposed to apply thinset to the floor and wall 2 inches in each direction, and then stick on the Kerdi tape. However, how do I deal with this vertical gap between underlayment and drywall bottom? Would it be silly to fill it with drywall compound and mimic the drywall panel being flush with the underlayment? Should I leave it alone as it will be covered up by baseboard later?
Leave the gap and cover it with trim. The gap is there to prevent the drywall from wicking moisture up from the floor and molding.
Leaving the gap should be fine. You will apply thin set up the wall and apply the Kerri tape. The small gap will be fine.
Later when you apply tile or baseboard it will all be covered. Whichever material you use will act as the rigid wall to the floor.