Create a Cube in blender, assign one material to each side of the cube there. Export mesh as .gltf with materials set to "placeholder".
In Godot You will have a MeshInstance of your cube. You can choose materials property in editor, and You will see 6 surface slots. Create new shader material there, new shader, with this code:
shader_type spatial;
uniform sampler2D image;
void fragment():
ALBEDO = texture(image,UV).xyz ;
didn't test it, I hope I didn;t make syntax error :)
Anyways, now when You choose any of your material, You will see new property image
under shader parameters. Input your texture there, and it should work.