doesnt matter if you use gypsum or hardiflex kasi isa lang yung floor mo between the rooms. useless yung walls kung hindi ka gagawa ng riser for the floor at isa pang internal ceiling. yung floor ang magseserve na medium para mag travel yung sound
as much as possible, dapat minimal yung contact nung inner walls, floor at ceiling mo dun sa outer room.
suggestion ko lang, if you are really concerned about your neighbors, buy an electronic drumset like roland vdrums. kasi for the cost of sound proofing, you'd be able to afford 100k worth of equipment.
-back to my previous suggestion-
my drum room is not sound proofed. i used to regularly play from 6pm to 7pm on weekdays. on weekends i play around 3pm to 7pm. my neighbors got used to it so i dont get complaints. id never hit the drums full on during regular hours and never past 8pm. id put rubber over the heads and do not use cymbals if and when i really want to play.