I prefer him as tank'gath
Build: Start with Doran's ring>Merc Tread>Mejai's/Leviathan>Leviathan/Mejai's>Force of Nature>Warmogs>Sell Doran's and get either Archangel staff or Atma's Impaler, had a test with Nashor's Tooth and works fine with feast/vorpal spike combos. Made lots of tweaking with this build. At both 20 stacks he's really strong, Rupture works fine against weak champs, Feast 3lvl nukes like hell, vorpal spikes does lots of damage combined with atmas impaler and hp, with this build he's fully capable to tank. But this build works as long as your team is able to kill.
Other build: Mejai's/Leviathan traded for sunfire cape and abyssal scepter(this one depends on who dominates in enemy team, if AD champ than take thornmail, if AP champ than scepter)