thanks for contacting us. The process for adding gaps rather depends on which style you are editing. For most styles it's as easy as editing padding values for the segments:
But for some styles this may interfere with the calculations being used. In that case it will depend on the particular style being used. Ideally I'd like to amend our flooring styles to include this feature and create version that have no hard coded values so that you can add your own geometry and the style automatically adjusts. When these were made we didn't have the features required to do this, but it's possible now. I'll add it to our wishlist.
For railings, the easiest way is probably to edit the existing segments. To to this:
1. Open the Style Editor
and find the Segment in the Default
2. Click the Extract button
to add the segment's geometry to your scene.
3. Edit the geometry to include extra horizontal geometry
I hope that helps, please let me know if you have any further questions.