With Gragas a lot of the time you will never get full build ( its about 40% chance you acyually will in my opinion, unless your opponent feeds you hard then you probally will finish yout build. anyway gragas is a great Ap mid and a great solo top Vs 2 people cuase of the way he farms.

really easy farming all you need to do is be able to aim a skill shot
if u cant reach the creeps

Make sure when you use his ulti that u push towards your team not away or it defeats the purpose of his ult