I'm stupid could someone explain why grasp is so good on gangplank

It's the melee value and he can apply it from range.

The Resolve tree in general gives him HP. Bone Plating helps reduce some of the damage he takes from early trades. Second Wind negates early trades or he can use Chrysalis to give him more early HP then turn that HP into AD later on and Revitalise increases the healing from his W (especially when he's low).

Grasp when procced, gives him more HP, gives him a little extra damage and heals him slightly. He can farm minions then Q a champion as soon as Grasp's timer finishes ticking to proc Grasp as his Q applies on hit effects and is coded as an auto attack.

Grasp Gangplank is incredibly obnoxious and is more obnoxious against melee champions because he can continually get Grasp procs off for free from range. The tree in general and Grasp pretty much negates his early weakness.

Ease of stacking.

Grasp of the Undying: Dealing or receiving damage within 2 seconds generates one stack every second. At 4 stacks, your next basic attack within 6 seconds, against an enemy champion deals 4% of your maximum health bonus magic damage, restores 2% of your maximum health, and permanently grants 5 bonus health. The empowered attack duration refreshes whenever dealing or receiving damage.

Gangplank's Q (Parley) will proc grasp on the enemy Champion, thus dealing a decent amount of damage and healing. I believe it's only good because they nerfed Kleptomancy into the ground.