So, this is my second build and i'm not stealing anything. There are 2 builds: Ap - off tank and full tank items. I hope you will like the build :)

Ap-off tank.
is best for AP casters, it penetrates magic's resistance, so you're doing more damage.
is for survivability, and how i said for off tank.
. Casters using spells. They wanna spam spells, this helps to.
Greater Quintessence of. same as seals.
Tank.
Greater Mark of Magic Resist and Greater Mark of Armor are ideal for tanks, more ressistance and armor, less damage from enemies.
Greater Seal of Evasion. This is important because sometimes (example. evelynn) will stun you with her E spell you can block it and you have chance to survive and to kill her.
Greater Glyph of Magic Resist.-||-
Greater Quintessence of Health. more HP, bigger chance to survive. Its important every hp in the teamfight.

Masteries are similar in both guides. I'm getting for AP Cho'gath 21 defence points for [Ardor] and for survivability. Tank is tank:) defence ...

When i say Survivability it makes me more on HP than magic ressistance or armor. You will spend more money for them(res&armor).

Rupture is very good CC spell. I'm growing it 1st. I would get first Vorpal Spikes for tank because i have teammates who have CC so i can do more damage.

Cho'Gath is slow champion. I'm getting for gank, help on teammates and run from enemies.
is spell for activate when someone diving the turret or attacking it.
. AP-caster needs it especially for Cho'Gath because he have low mana and his Spells are spending too much.

You'll farm minions with E passive spell, keep that active and you will attack more minions. At lvl 6 use Feast on Minions till 6 stacks. IN Late game if you have 6 stack on feast try to use the ultimate on Enemies. Cho'Gath has a good combo: IF you are in grass, use Rupture on the champion and if he jumps go and use W spell( Feral Scream).After few normal hits use Feast on him and he will die(just first read the true damage which ultimate does and target's HP).
Playing as tank is very simple:) Go first and try to CC enemies.

ENJOY THE BUILD :)