OK, the main page lacks information.

https://www.presslabs.com/gitfs/docs/usage/

https://www.presslabs.com/gitfs/docs/arguments/

So apparently you can mount remotes, not only local .git dirs. This is really handy and it should be listed on the main page, really.

I raises an other question: can I mount remotes where I don't have write access read-only?

For the directory structure the history directory makes sense for mounted projects that are edited with gitfs. For other projects, not so much. What about having a read only tags/ and commits/ directory too?