Using Smite upon the Gromp will grant Gift of the Toadstool for 90 seconds, which poisons enemies who attack you: dealing 10 + (10% of bonus health) magic damage over 3 seconds. This bonus can only be claimed once per spawn.

Usage:

Helps with clear speed and wave clearing at all stages of the game, though the impact increases considerably as you gain bonus health.

Randuin's Omen (+500 HP), Banshee's Veil (+450 HP), Warmog's Armor (+800 HP), Enchantment: Cinderhulk (+350 HP +25% Total Bonus Health)

With those items alone that offers you 2625 Health and 10 + 10% of that is 272.5 Magic Damage delivered over 3 seconds so 90.8 Magic Damage/per second.

For comparison at Rank 5, Nautilus's autoattacks due to Titan's Wrath only deal 35 Magic Damage/per second if they haven't built any AP.

The damage is actually kind of insane late game.

The upshot is, leave Gromp for your Tank Jungler when you think a team fight or fight for control of a major objective is going to break out and give them time to go an get it when feasible.