Hi Don,
IFC standard file (IFC-SPF) follows the definition of the STEP-file defined in ISO 10303-21. From what I understand is that the two formats should therefore be interchangeable. If not, most CAD-type software that can export STEP files should also be able to export IFC formats.
Since the original question is: How to convert .step / .stp to GLTF, I just pointed to an IFC loader that I know works (it’s still a work in progress, but most of the features are there) that works with Three. So what about you load your model with the ifc-loader, then export the scene with the GLTF exporter? I think that answers OP’s question, right? Or at least give some direction of where to look for something that could be possible.
At the very least it should give a starting point of building a STEP loader if someone wants to give that a shot 