As gwen, If they invade you early, just back off. This is especially true if they are ahead, as you will not win early fights with them.

Be proactive with your Ultimate. Force ganks with your R!

As they will be ganking often in the early game, you need to make sure that you are also looking to gank as often as you can and abuse your E.

Focus on securing those objectives. You must take Rift Heralds and Drakes whenever you can!
 As gwen, If they invade you early, right back. It's especially true if they are early, because you won't win early fights with them. Be proactive with your Ultimate. Force the ganks with your R! As they will often glove in early play, you need to make sure that you also look at gank as often as you can and abuse your E. Focus on securing these goals. You need to take Rift Heralds and Drakes when you can!