This should fix it
1 - Does it display correctly if you open exported Gltf model in these Gltf viewers:
2- Make sure the model material is setup as explained here
https://docs.blender.org/manual/en/2.83/addons/import_export/scene_gltf2.html
and
https://wiki.jmonkeyengine.org/docs/jme3/external/blender/blender_gltf.html
3- You might need to add a light probe to the scene. (though this is not necessary)
If you do not know how to add it, then follow this topic: