GW boots emunand just fine. You have to be launching it from a 9.0 firm. If you're one of those people who decided to update your a9lh sysnand, and use a 9.2 emunand to launch Gateway with, then there's your issue. In that case, Gateway is trying to launch 9.2 using 10.x firm when you tell it to boot emunand.Gateway for unknown reasons to me can only boot Gateway Mode (sysnand) [hold B when choosing GW mode from MENU] and black screens booting GW Emunand mode.
I keep the standard 9.2 sysnand, with a fully updated emunand. My emunand was made through the GW menu (in case something with emunand 9 breaks it).