I dont see the usefulness of lich bane fro gragas, IMO is wasting money that could go to rabbadons, considering that body slam also scales from AP i dont see that much benefit

I suppose it is a little bit of a Hybrid. I get a Mejai's when I play in a premade, and can for certain trust my allies. Its not an occasional build in my opinion or my experience however. Gragas scales amazingly well, .9 for his barrel roll, .5 for body slam, and 1 for explosive cask, so almost 300 AP by your fifth item with decent Magic pen will make you quite formidable.
I do build up his off-tank nature somewhat, but I think if you are looking for a champ who focuses exclusively on spells I would work with a more mainstream caster. Gragas is one of the coolest off-tanks because he has the spell range of regular mage for some situations, and at the same time a definite melee presence for others. To not take advantage of that wouldn't do the drunken fool justice, imo.

Quite a hybrid build. Not so AP and neither not too Tanky/AD
Will work but it's very occasional.

This is a pretty decent guide. Personally, I would add more to what tanking items are good for him. Make sure to mention Banshee's Veil! It is probably the best item to counter CC for EVERY champion.