Gwen Runes explained
There's one set of runes for Gwen on patch
11.15
that is the clear favorite for LoL players and pros alike. The best Gwen Runes on the position 
Top are Precision as the primary path and
Resolve as the secondary path with a pick rate of 94%. Besides runes you should take a look on Gwen Build and if you are interested in pro matches take a look at Gwen Probuilds. If you want to discuss
Gwen then you should visit the champion subreddit.
Primary path on Gwen
Precision improves attack and sustained damage on Gwen troughout the match. Most of high elo players – and our LoL pros in the the major regions – have agreed that
Conqueror is the best keystone rune to run on
Gwen. This keystone rune provides additional power in the later stages of a game. As soon as
Conqueror is fully charged, Gwen deals additional damage and gains life steal. This is usually followed up by taking
Presence of Mind,
Legend: Alacrity and
Last Stand.
Presence of Mind helps Gwen with the mana problems by restoring missing mana, if a champion is killed.
Legend: Alacrity grants additional attack speed to Gwen. With
Last Stand you deal more damage while you are at low health.
Secondary path on Gwen
When it comes to your secondary path, most
Gwen players prefer to go with
Resolve.
Resolve empowers the survivability of Gwen and increases the tankiness. On the
Resolve path, the best runes for Gwen are
Bone Plating and
Unflinching.
Bone Plating make Gwen overall tankier, by decreasing the amount of poke damage in the early game.
Unflinching inceases Gwen tenacity based on your missing health.