I would check the metalness, if metalness is too high it cannot react to light but only to environmentmaps, which you most likely do not have. Re-export the model with reduced metalness and/or add an environmentlight. I had the issue of black gltf models with PBR Material as well. This code pretty much solved my issue (also had to download the .dds-file):
scene.environmentTexture = new BABYLON.CubeTexture.CreateFromPrefilteredData('textures/environment.dds', scene)