X and Zero also had their mysterious ghostly creators step in regardless of previous restrictions, and somehow repair them in full.
Gate had his CPU/Control chip damaged, most likely, otherwise no matter how much damage he took, he could possibly be repaired or rebuilt. Remember, in X1, Zero lost his physical body, but it was still possible to bring him back because his control chip miraculously survived destruction. In fact, the only reason they didnt, was because no matter how hard he tried, Cain could not rebuild Zero.
The same reason iris died. Her chip was damaged by her becoming the ultimate soldier. When Sigma shot his energy beam at Gate, he basically fried his systems. Plug your alarm clock into the wrong adapter and see what happens.
Gate was fried pretty bad. Therefore while it may not be impossible altogether, He was pretty much beyond repair. And if they did, he might not be the same Gate as he was before.
And Douglas does not repair Reploids. (that we know of) He repaires the Machinery they use. things like Land chasers, Ride armors, Weapons, etc. Not them themselves.
and I dont think he will appear in ZX. I dont thik he would really have any place there either. Any ties he had storywise that could warrant a return for him, died with Sigma and Mother elfs creation. He really has no purpose after that.