Gronk is a pet acquired through Herblore. The chance to double potions from Gronk is additive with the bonuses from Herblore Mastery, Aorpheat's Signet Ring and Agility obstacles, and multiplicative with the item doubling effect from wearing any of Herblore Skillcape, Max Skillcape, or Cape of Completion.

This page is up to date (v1.2.2).
Pet ID: melvorF:Gronk
Source: Herblore
Drop Chance: See: Acquiring Pets
Effect: +2% Chance to Double Items in Herblore
Part of 100% Completion: Yes

There is no best potion to brew to acquire Gronk, all potions are equivalent.