DIAGNOSIS The dispenser drawer is likely locked because a cycle is in progress. The GTW840CPNDG has an electronic lock for the drawer during fills and washes to prevent adding items mid-cycle. If no cycle is running, hardened detergent or fabric softener residue has glued the drawer slides shut.
FIX First, ensure the machine is not running. Press 'Start/Pause' to cancel any cycle and wait 2 minutes for the lock to disengage. If it remains stuck, pull the emergency release tab located at the top center of the dispenser housing inside the cabinet. Simultaneously pull the drawer straight out with firm, even pressure.
IF THAT DIDN'T WORK Remove the drawer completely by lifting the rear tabs once it's partially out. Soak the entire drawer in hot water to dissolve residue. Use a toothbrush and hot vinegar to scrub the drawer cavity and the chutes inside the machine.
HEADS UP Using powder detergent or more than 2 tablespoons of liquid HE detergent is a primary cause of this clog. Run hot water through the empty drawer monthly to prevent buildup.