Say Gapejaw Bog stadium is in play and Ting-Lu ex with "Cursed Land" Ability is in the Active Spot. If I put Pumpkaboo onto the Bench, can I use its "Pumpkin Pit" Ability to discard Gapejaw Bog before it places damage counters on Pumpkaboo?

Since Pumpkin Pit and Gapejaw Bog are simultaneous effects, it is up to the player to choose the order in which they will be applied. If the Stadium discard effect is resolved first, no damage counters will be placed on Pumpkaboo. However, if the player chooses to put the damage counters from Gapejaw Bog first, then Pumpkin Pit won't work because Ting-Lu ex's Cursed Land would shut off Pumpkin Pit.

Relevant cards

SWSH10_142

SV2_127

SWSH7_76