Gragas Runes explained
There's one set of runes for Gragas on patch 14.23 that is the clear favorite for LoL players and pros alike. The best Gragas rune paths on the position 
Top are 
Sorcery as the primary path and 
Inspiration as the secondary path with a pick rate of 63%. Besides picking runes on Gragas you should learn how to play Gragas and if you are interested in pro matches take a look on how a pro builds Gragas. If you want to discuss 
Gragas then you should visit the champion subreddit.
Gragas runes from primary path
Sorcery empoweres abilities of Gragas and increases the available champion resources. Most of high elo players – and our LoL pros in the the major regions – have agreed that 
Phase Rush is the best keystone rune to run on Gragas. Phase Rush offers more flexibility and options to betake oneself in and out of danger, which can be helpful since that's essentially the main playstyle of Gragas. Other primary runes on Gragas are 
Manaflow Band, 
Transcendence and 
Scorch.
This is usually followed up by taking Manaflow Band for additional mana and mana regenaration. Transcendence for some more Ability Haste. Scorch increases your control in the early game.
Gragas runes from secondary path
When it comes to your secondary path, most Gragas players prefer to go with Inspiration. Inspiration opens creative opportunities on Gragas troughout the match. On the Inspiration path, the best runes for Gragas are 
Cosmic Insight and 
Biscuit Delivery.
Cosmic Insight grants summoner spell and item haste. Biscuit Delivery is a great rune for laning as it gives additional health and mana, which can help in more aggressive lanes.
FAQ for Gragas Runes
Which is the best keystone rune on Gragas?
The best keystone rune on Gragas is Phase Rush. This Gragas runes setup is then referred as Gragas Phase Rush.
What runes does Gragas use?
Runes from the paths Sorcery and Inspiration are the most used runes on Gragas. For the exact pick rates of Gragas runes used, you have to study the rune distribution presented.