Ho-o-oly shit that's amazing. Thanks for finding this Jei! Definitely gonna have to make use of this. Best part is, unlike EX Cancelling it it seems you can burst out of a summon cancel.
Here is the vid, I know that you already noticed that in can be donde with Omni dash, but just to get sure XP
Call me slow but I don't wholly understand the full use of this...You use a summon while doing HGC to do...what? Move around freely after or getting a summon off directly after an attack..?
The idea is that you use Hell's Gate and make sure it connects to the ground, it stores some sort of Hell's Gate Counter, so when you dash up to someone, both you and the opponent must be on the ground or slightly off, and then you guard and immediately summon, the game will cancel the block due to the summon, and Hell's Gate will be used instantaneously.
It's an alternate form of Hell's Gate Cancel which relies on the use of a Summon, instead of a full Ex-Mode bar.