If you test this model in a standalone glTF viewer (https://gltf-viewer.donmccurdy.com/, https://sandbox.babylonjs.com/) I think you’ll see the same result — that the materials have not been exported with the file.
Blender’s material system allows pretty complex arrangements of nodes, must of which cannot be exported to other software. To get the model to export to glTF, you’ll need to review the Blender glTF addon’s documentation: https://docs.blender.org/manual/en/2.82/addons/import_export/scene_gltf2.html
In particular, you’ll want a Principled BSDF material, or perhaps an Emissive material. You’ll also need to bake the gradient into a Texture. Blender has tools for that, but I’m not an expect on texture baking.