if he stacking attack speed, then hes dependent on his E for damage....he'll activate it when he jumps you...kite him for 5 seconds and you'll be stronger than him afterwards.....
if hes stacking damage instead of attack speed, get thornmail
if you're saying he dives you at 6, its more likely that you weren't kiting him(Autoing him while going back) (sorry i can't tell from reading you're description nor did i see the game)
think of him has meele version of vayne running up to your face to do damage.....he'll do alot of damage, but if you hit him back...you can kill him really quickly...but you HAVE to attack him AND walk backwards to reduce his dps to you. I've seen alot of inexperienced gnar players that go mega gnar and never auto attack but just right click away
as yi, the enemies that are scary to vs are the ones also glass.....like glass vi, xin.....even somtimes good gnar players that choose to start off with botrk... it forces yi to not go glass since you can blow him up before he can even do anything forcing him to play tanky resulting in an increase of time for yi to kill his enemies. Which simply means, his 10 second ult may not be enough to kill somone everytime.
a yi without ult is a dead yi(unless hes super fed), he does half the damage and has no escape+can be kited easily
but its also fair to say i never have problems against a gnar when i lane as yi
good luck!
