This builds maximise clear speed and burst damage early game, and transition into more tanky mid and late game. You still have 500 AP and 25 magic pen from the runes and items even before buying the 6th item. You can OS squishy and do big damage to tanks and clear camps and minions fast, but you also have tons of AH (50), tons of HP and resistances.

Lich Bane gives you AP, AH, MS, and a sheen passive, allowing you to clear camps fast and move around the map fast, with big burst ganks. Ideal 1st item.

Rabadon 2nd, in season 14 Rabadon has big base AP and lower scaling AP%, making it better 2nd item as it used to be. You have 100 bonus AP from runes, and with Lich Bane and Gragas AP ratio, this is a must 2nd item.

Now you're getting into mid game and grouping with your team. You have about 500 AP 25 magic pen. You don't need more damage, trust me you got plenty. But you WILL win more games if you have more survivability, you won't get OS and you'll be able to tank some damage in teamfight and do more CC to help your team. Gragas has built-in resistances on W, so what you need is HP and AH. Two items give HP and AH, anathema and fimbulwinter. They are both great and cheap, but Fimbulwinter takes time to stack... just get Anathema and use it on the damage source of enemy team. Then you got tons of HP, making Jak'Sho very valuable. Now you're very tanky, and you can last long in fights, making Riftmaker a great purchase as last item, giving you more AP HP AH and a healing passive