I know there are a few Trek owners and potential buyers that are on the fence about a SRAM (often GX) build vs a Shimano (often XT) build or upgrade.
As of yesterday Nov, 26th 2019 the W586894 end cap is listed in Trek's system as out of stock until March 16th, 2020. I saw the numbers on the screen at the Trek LBS yesterday, and I was on speaker phone with the Trek rep when they called to verify and ask about an alternative part number. "No" was the answer.
My other attempts to find a non-Trek alternative have failed so far, as well as my attempts to find a Trek dealer with any stock. I asked for the source of the hub, but so far the answer has been "this is a Bontrager part, and has nothing to do with DT". Based on Trek/Bontrager's previous re-branding of hubs I find it unlikely that they are all completely Trek. On the other hand, just because they are possibly DT, I9, Shimano, Sram designed/sourced doesn't mean that they aren't proprietary.
I played the fool and ordered a Shimano XT drive train with plans to swap my 2020 TF GX parts to another GX bike with a worn out drive train (Trek Stache). It looks like I'm returning that stuff and ordering a GX drive train instead. Unless I can figure this out in a reasonable time frame.
If anyone has any information about an alternative end cap, I'd love to know. I'm also pretty confident I can make something work if I know the OD at the hub end and the overall length. The micro spline cap is different in these two respects. I know it is longer, so if I could find a longer end cap I can modify my way to the correct dimensions.