I can't reproduce that result. There are some small elevation changes on the aprons, but nothing like in the screenshot.
Maybe another addon that interferes here?
Was there ever an update to resolve this issue? I have just bought the scenery from JustFlight and have the same issue as the OP. I have opened the content.xml but the only entry in there is for navigraph. Thus, how would I sort GSX above airport sceneries?
The above solution referred to an older version of MSFS. These days you use the Package Reorder Tool to change scenery priorities.
It should be sufficient to go into the GSX scenery folder and remove the bgl file for DTNH though. No need to rearrange the addons.