
Patch 7.3a

Favorable matchup
Fiora has the tools to contest Gwen's duel: lateral mobility, Vitals and Riposte for a big chunk of damage. The mist doesn't make Gwen invulnerable to an opponent who steps inside it, but it does give her resistances.
The center of Gwen's first ability is the especially dangerous part. Fiora's first ability lets you threaten a Vital from the side and reposition so you don't take that whole line. Going in head-on against ready scissors wastes your mobility.
Riposte can answer scissors or a volley of needles if it lines up with the impact. Since those abilities aren't immobilizing effects, blocking them doesn't by itself turn the thrust into a stun. Against Gwen you should also weigh Riposte's attack speed slow.
Look for a Vital when Gwen has used her first ability or doesn't have her scissors charged. If she has already used her mist, the next trade is easier to set up; don't stand still in the center to finish the duel.
As Gwen, protect the Vital side with your positioning and keep mobility to adjust the center of your scissors. Don't feed a big burst of damage into Riposte: fake the trade and punish once she uses it. Inside the mist a nearby Fiora can still hit you, so don't rely on it as immunity to the duel.

Favorable matchup
Jax can deny a big part of Gwen's trade with Counter Strike and end it with a stun. He has to avoid the center of her scissors and the needles: dodging basic attacks doesn't remove her true damage or all of her healing.
Gwen empowers her attacks after her third ability and uses them to charge her scissors. Counter Strike denies basic attacks during its defensive window and threatens a stun. It doesn't make scissors that were already charged disappear or automatically block all of her abilities.
The stun lets you land your attacks and end a trade without Gwen chasing you freely. Keep a leap target to get out if she uses her ultimate. Fighting next to her inside the mist is possible, but it doesn't cancel the resistances she gains there.
Trade when Counter Strike is ready and Gwen has few scissor charges or has just spent them. If she starts her ultimate sequence, prioritize dodging and getting out before contesting the duel again.
As Gwen, avoid wasting empowered attacks into Counter Strike and use your abilities from an angle that doesn't force you to chase his leap. If he comes in with all his defenses up, disengage until it wears off. Once Jax no longer has Counter Strike, your attack window is more useful, as long as you're not exposed to reinforcements.

Execution matters
Shen can limit Gwen's empowered basic attacks and contest short trades with his shield and blade. He's an answer built on containment and team utility, not a tank who should soak all of her scissors and needles head-on.
His second ability protects a zone from basic attacks, reducing the value of Gwen's attack window. Her scissors and needles are still threats. Blocking basic attacks also doesn't remove scissor charges she earned before the trade.
Pulling the blade through Gwen empowers Shen's attacks and helps contest short contact. His passive provides a shield against part of her response. Your goal is to hit and separate; stacking lots of health doesn't neutralize Gwen's health-based damage and her true-damage center.
Look for a trade with your blade and Spirit's Refuge available when Gwen has used her first ability or has few charges. If she still has her ultimate for a long fight, keep your third ability as an exit.
As Gwen, don't waste your attack window inside Spirit's Refuge. You can threaten with scissors from a favorable angle, but track where the blade is to avoid an empowered Shen trade. If he uses his ultimate away from lane, take advantage of the wave and call out that he left; don't follow him late along a route without vision.