I have finally found some time to look into this, and turns out, it's
not a problem anymore (with 18.04 at least). If I set the local user
back to normal (so it does appear in the greeter), GDM works normally
(it will display the user list, without any freezing). Of course, you
cannot log into the local user, because the home directory is simply not
there (the remote /home is mounted onto the local /home).
As a side note: when you do attempt a login into the local user (with
the correct credentials), you will not get any error message (apart from
the journal), which is not a very user-friendly solution - GDM just
restarts the greeter.
When I try to login using the terminal, the login will work (as it
always did - also with 17.10), but bash will complain, that it cannot
chdir into the home directory and access its stuff there.
So again: I posted this bug in October 2017, and I did not know then
where to properly post this and now (with Bionic) I can't reproduce the
problem. It probably was not a bug with autofs. What's relevant for me
now is the root_squash issue here: #1733101
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GDM3 hangs, if local home directory is not accessible
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