Why Mid > Top
It basicly comes down to why is assasain Gwen better than bruiser Gwen.
A lot quantitative reasons can be listed: mid quest being op, better matchups, lich bane buffs, etc..
But My take is that what makes her so good comes down to Gwen’s best ability, hallowed mist (W)
W duration is 4 seconds But acts like 2 completely diferent abilities in a fight, these parts i like to call:
GII : as in gwen is inmune.
GIP : as in Gwen is protected, since She gets some def stats during this part.
The 4 seconds are split into GII and GIP depending on when does the enemy champion enter the mist. I think is pretty clear that during a fight Gwen’s objective is to maximize GII.
So W “scales” with how much damage you can do during de GII because If u get them down to 20 % hp during this time they have to back up, on the other hand, If they manage to enter your mist healthy Its over.
Is Kinda the same reason as why Sion would not utilize Kayle’s R as efficiently as She does, is not the second of invulnerabiltiy whats op, is what She does during that time.