What runes and items would you pick for Gwen?
For runes, I'd personally pick:
Conqueror - Extra damage and good sustain. May synergise with the Q if the snips proc multiple Conq. stacks.
Overheal - Synergises with the Passive really well. Grants some extra sustain.
Legend: Alacrity - On-hit builds and good damage output.
Legend: Bloodline - Extra sustain maybe? Can synergise with Overheal and Passive. Use this if you're probably getting burst to literal death.
Coup de Grace - Good to finish champions off, especially with the AP burst the champ offers.
Cut Down - Great for health-based tanky enemies like Cho'Gath or Mundo.
Last Stand - Good for finishing off enemies on low health since you are quite squishy.
Sudden Impact - Your E can proc this really well. Useful against armour-building tanks like Rell.
Ravenous Hunter - Synergises with Overheal, Bloodline and your Passive. What are you, a healing machine?
Attack Speed - Do more DPS
Adaptive Force - Without it, you're a dud.
Armour - Good sustain. Get magic resist if AP-heavy team.
For items, I'd pick:
Riftmaker - Good sustain with one item. I'm not gonna mention this again.
Night Harvester - Good burst and movement speed that allows you to gap-close enemies easily. Bye-bye health bar.
Galeforce - My personal pick. Good gap closer and grants you a nice double dash when used in conjunction with your E.
Sorcerer's Shoes - Makes your scissors good.
Plated Steelcaps - For the pokers. I'm looking at you, Quinn.
Nashor's Tooth - Enhances the passive even more, just like how Viego enhances his Q passive with his Blade of the Ruined King.
Blade of the Ruined King - Insane synergy
Cosmic Drive - Go fast.
Demonic Embrace - Burn.
Chipmunk Chainsword - Good heal negation.
Morellonomicon - Same.