My question is about opening .glb or .gltf files in Blender. I don't see these extensions in the Import options, although Export allows me to create files with either of those extensions.
Blender comes with glTF-Blender-IO pre-installed since 2.80.
This is a refactoring of the original Khronos glTF exporter, combined with @Julien's importer. Both of the original projects should be considered obsolete in favor of the newer, combined glTF-Blender-IO project.
The Blender Manual includes documentation on ways to use glTF.
GLTF Importer for Blender - now in progress:
A more advanced glTF importer for Blender : https://github.com/julienduroure/gltf2-blender-importer
In short, there's an exporter for the GLTF format from Blender (which I assume you have installed), but as of this moment, no one has written an importer add-on or extended the export add-on to support importing as well. This is why you can export, but have no options for importing.