[Damienthathedge]

I actually have the Gashat myself and I think that it's gonna work like this.

Driver closed, pop in Maximum Mighty, latch on Hyper Muteki then open the driver and click in the Ex-Aid into the gashat and the button on Hyper Muteki (I'm assuming this is why the button Maximum Mighty has a giant A on it). Or another thing could be that once you press the button on Hyper Muteki the little black button gets pushed in and the gashat itself just latches onto the plastic.
Judging from the scans we saw I think the Hyper Muteki gashat itself folds open to cover the Gamer Driver screen.

The video forgot to show off this what I'm assuming is a re-activation button, if you press the button again a weird "activation" noise gets played and the gashat starts flashing colours, could also be a Critical attack.