The only special thing that GregTech does is that ores are tile entities and the server does not send the tile entity data to the client until the ore block is actually visible (to save on bandwidth). The client still knows there is an ore block, it just doesn't know which ore it is. XRay version 39 is quite capable of viewing these ore blocks. It's only XRay version 47 where it is no longer capable of doing so. I do not have any inbetween versions to test which exact version broke this.
That's very interesting. I'll see if I can look into this further.
Also, once school settles down I will be releasing an update for the XRay mod. I have it mostly working with the latest Minecraft version and just need to make some time to add the finishing touches.