Gloomhaven · Board game rules
When do traps trigger disadvantage on attacks in Gloomhaven?
Quick answer
Traps themselves do not trigger disadvantage on attacks — disadvantage comes only from being an adjacent target of a ranged attack or from specific abilities/conditions, not from traps or hazardous terrain.
Details
The rulebook does not describe any rule where springing or standing on a trap causes disadvantage on an attack. Traps instead inflict damage or another negative effect and are then removed from the board when a figure enters their hex with normal or forced movement [p.14 — Scenario Setup]. Flying and Jump movement ignore traps entirely [p.14 — Scenario Setup].
Disadvantage is a separate mechanic that applies in specific cases: any ranged attack targeting an adjacent enemy automatically gains Disadvantage for that target, and certain character or monster abilities/conditions (such as MUDDLE) grant Disadvantage [p.20-21 — Advantage and Disadvantage; p.23 — Conditions]. When an attacker has Disadvantage, they draw two attack modifier cards and use whichever is worse [p.20 — Advantage and Disadvantage].
Monsters do factor traps into movement decisions in a way that can relate to avoiding Disadvantage on ranged attacks: monsters without Flying treat trap and hazardous terrain hexes as obstacles when pathing, and a monster will move away from its focused enemy until it can perform a ranged attack without Disadvantage — but this is about adjacency, not the trap itself [p.30 — Monster Movement; p.31 — Monster Interaction with Traps and Hazardous Terrain].
Watch out: Don't confuse "entering hazardous terrain/traps" (which causes damage) with "attacking an adjacent target with a ranged weapon" (which causes Disadvantage) — these are unrelated triggers that happen to both involve terrain and combat.