I have imported a Gltf file into blender. I setup a sun light and rendered it with eevee. it works absolutely fine when rendered with eevee, but in cycles some part of the plane becomes dull with a different intensity.
A link to your GLTF file is not helping so much because we don't know what settings you used to import it. So I imported it with the default settings.
After that I saw the same pattern, I selected a front face and deleted it, there was another one and so on - the front side had 6 layers of faces overlapping in the same place as suspected.
To avoid this, you should enable the option Merge Vertices when importing the file. In this case Blender checks if there are multiple vertices/faces in the same location and merges them.