What's the legendary drop rate of Ghom?

The legendary drop rate of Ghom is different depending on your difficulty level. Most people farm bosses in normal mode because of the checkpoint system. If you're looking for legendary items, you get a much higher increase while in a rift, so you might want to consider elite farming a rift instead.

I've included the % Bonus for difficulties below for you.

Normal: 100% Health, 100% Damage, 0% extra gold bonus, 0% extra XP bonus

Hard: 200% Health, 130% Damage, 75% extra gold bonus, 75% extra XP bonus

Expert: 320% Health, 189% Damage, 100% extra gold bonus, 100% extra XP bonus

Doubled bounty currency rewards for Expert and all higher levels.

Master: 512% Health, 273% Damage, 200% extra gold bonus, 200% extra XP bonus

Imperial gems may drop instead of Marquise on Master and all higher levels.

Torment I: 819% Health, 396% Damage, 300% extra gold bonus, 300% extra XP bonus

Torment+ enables class Set Items at lvl 60+, and at lvl 70+ possible Key and Organ drops, plus increasing bonuses to legendary drop rates and extra bonuses to legendary drop rates in Nephalem Rifts.

Patch 2.0.5 enabled Torment-only items to drop from horadric caches earned on T1+.

Torment II: 1311% Health, 575% Damage, 400% extra gold bonus, 400% extra XP bonus

Patch 2.0.5 added a legendary drop rate bonus for horadric caches earned on Torment 2 and higher.
Torment III: 2097% Health, 833% Damage, 550% extra gold bonus, 550% extra XP bonus

Torment IV: 3355% Health, 1208% Damage, 800% extra gold bonus, 800% extra XP bonus

Torment V: 5369% Health, 1752% Damage, 1150% extra gold bonus, 1150% extra XP bonus

Torment VI: 8590% Health, 2540% Damage, 1600% extra gold bonus, 1600% extra XP bonus