Is it close to elites? Is it worth to do Ghom repetitively? And in that case, should I do it in adventure mode or normal mode?
Most people that I'v eseen run it does it in story mode as the waypoint is right next to him. As for drop rate, he's considered a 'boss drop' which gives more loot than elites, thus more chances at legendary items.
Most people who does this run wears a Mara's Kalideoscope which completely nullfies his poison damage. I suppose the advantage is greater for HC players since there is zero chance of dying, even on T6 when wearing that amulet.
Whether its worth it or not is pretty subjective I'll leave that up to you to decide.
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