This workflow really depends on what is in your CityGML file and what the geometry types are. But in general you should be able to just add a reader (merging all feature types). You can use a GeometryFilter to filter out null geometries and then all other geometries into the GLTF writer. In the GLTF writer, set the version to be version 2. You can also check Move to local coordinate system (this will make sure your gltf model is is not in the middle of nowhere when using the model in another application.

This is a pretty crude approach which will generate a separate internal mesh node for each feature but is a good starting point. In the end for best performance you'd probably want a single mesh in the gltf which can take a little more work to achieve.