Both plugins use libacfutils (An aircraft plugin library) XGS is using an older version of it, and it is loaded before our aircraft, so X-Plane decides not to load our version (since one is already loaded by XGS) but since it is an older version, it is not compatible. We are working on implementing a version of libacfutils that is linked statically with our plugin, so it is forced to load again, but that is still being worked on.
Until we can get a static version of libacfutils working with our plugin, I would recommend using a different landing speed plugin, there are many available that do not rely on libacfutils, here is one that I have used: https://forums.x-plane.org/index.php?/files/file/7995-landing-speed-plugin/