glTF materials define a "doubleSided" property, which disables back-face culling (see https://github.com/KhronosGroup/glTF/tree/master/specification/2.0#double-sided )
Currently it's not possible to set this property in Blender exports. Seems like other users need some solution for this as well:
This patch introduces a custom material property for glTF "doubleSided" and a UI Panel for it in the material property section.
I'm not sure if glTF addon patches are supposed to be submitted here (or their GitHub page https://github.com/KhronosGroup/glTF-Blender-IO ) or if this solution is in general a proper way of solving this, but I hope someone can clarify and give feedback.
Thanks for the consideration,