I'm sorry, but we can't obviously change the way GSX operates "just" because another product (which is not even an FSX add-on. ENB is DESIGNED FOR GTA4!) cause a crash to any window (not just GSX one, according to your report) that gets opened.
The 2D gauges are entirely different way of drawing windows, they are related to DirectX, that's why they might not cause problems with ENB.
The GSX Windows are not even opened by GSX, which only use standard/official Simconnect commands to request the sim to open a menu. And here, there's a difference between P3D and FSX. P3D use an entirely different menu/UI system and, in fact, has a bug when opening a menu, which cause the sim to lost focus unless you click on the menu again to process keys.
This doesn't happen in FSX, it's something specific to P3D, Not sure if this has anything to do with ENB, but it might, perhaps the different way P3D creates a menu, have an affect on ENB, I don't think the crash in the menu creation is just a coincidence.
We reported this bug to LM a long time ago, they acknowledged the problem but said it wouldn't be an easy fix, so we can only hope they'll fix it in the next version.