Your main focus as Gwen Jg is to clear and scale.
Playing on your item spikes is the best way to win a game.
DOnt try to really gank pre 6 ( can kill if you have a ghost and you have setup/they commited too hard )
The best option is just to full clear and insta recall to get a blasting wand then run back to your camps to complete your nashors tooth and get 6 as soon as possible.
Once this is done you can buy T2 boots and go for a deathcap if you re super ahead and a shadowflame if you re not that rich , you can pick up a dark seal if you have too much money or if you have the opportunity to take kills in teamfight ( if the team plays around you )
you need to complete a zhonya or the other of the two items ( deathcap or shadowflame)
you choose which one to go ( zhonya if too much burst / your Q and R will go off even you zhonya during it )
As last item you will want some % magic pen because at that point they identified you as an ap menace and they will rush MR ( biggest gwen counter)
So buy a void staff if they stacked mr or cripbloom if they just took a bit
You can also go rift maker last item if you front line and they didnt bought MR

Thank you for reading my Gwen Jg guide
Good luck on the rift