In order to be consistent on Gwen, you have to build some tankiness.
Rush Riftmaker 99% of the games.

Then, for your second item, see what you need the most right now :
If you need to survive, building tankiness might be the way : Randuin, Spirit Visage or Zhonya are good as 2nd/3rd items.
If you need more damage, you can go Nashor's Tooth, Morello (against lot of heals), or Cosmic Drive (against high mobility).

For your 3rd item, if you took a tank item 2nd, build damage. If you took a damage item 2nd, build tankiness. Simple as that.

The same thing goes for 4th and 5th item :
If you need more damage you can take Demonic Embrace against tanks or Rabadon to hard snowball. You can also take Cosmic Drive or Nashor's Tooth as your 4th item if you didn't build it and feel that you need it now.
If you need tankiness, Guardian Angel is really good if enemies just one shot you, Gargoyle to be a strong frontline in fights, Dead Man's Plate against fast champs, and Force of Nature agaist mages that are hard to catch and poke a lot.

For boots, Ionian's are OP rn, but you lose some tankiness, so build it only if they don't have a lot of autoattackers(Tabis in this case) nor cc(Mercuries in this case).

My friend had the idea to play Gwen, but he only wanted to play Midlane, so I tried my best to make a guide that would help him play a champ that he likes in this role.

Surprisingly, she's actually quite strong Midlane. She can destroy almost every weak early mage, such as veigar. What's so strong about Gwen Mid is her mobility coupled with her W, as she can avoid poke, dodge skillshots easily, and get on top of her enemies. Because of that, you want to abuse your mobility and strong damage against weak early game champs.

When playing top, you will encounter bruisers, such as Riven or Sett. They will almost always win agaist you, they just have the upper hand. But Midlane has a lot of good matchups, and almost every time you can do ok in your games.

YOU SCALE. If you're facing a champ like Lucian, Renekton, or Leblanc, you just have to wait until you're stronger than them, by farming and, most importantly, surviving. SURVIVE. If you die, you lose waves, your opponent gets stronger, and you're screwed.