GLTF 2.0 supports double sided material, however, even if a surfaces has a double-sided appearance in FME the output does data not honour this.
For reference, here is the gltf specification the double-sided materials can be defined.
https://www.khronos.org/registry/glTF/specs/2.0/glTF-2.0.html#double-sided
This is of particular importance because it can be really difficult to correct incorrectly oriented surfaces - being able to create double sided materials can help to overcome this challenge.
Here is an example workspace which shows the issue