Thanks for the suggestion. The reason why I used to go Hextech Gunblade was due to two reasons: The AD increases your ultimate damage by 140 which still deals magic damage, the active on the Hextech Gunblade scales with AP, thus acting like a replacement for Deathfire Grasp, and finally the ad helps with sinister steel damage. The AD gives you a mixture of damage when it comes to auto attacking too. Will of the Ancients I buy now due to the nerf on her resets, so capped cooldowns is a must now which I have already mentioned at the bottom of my guide. I also explained why Hextech Gunblade in my guide too being the 2nd last or 3rd last section :P

I like this guide, but i have a few suggestions to make:
1. Why Hextech Gunblade? Kat is a burst mage, why would you ever need a slow? You don't need that little AD either, especially when you are not buying armor pen. If you want spell vamp, get Will of the Ancients.
2. Put the matchups in spoilers, it's hard to scroll through all the matchups and find the one you're facing.
3. Turn on "comment to vote", or CTV, this prevents troll voters and helps give feedback.
Good job and keep on writing!