I don't need to have a gdm so I want to disable and remove it. Is that easy to do? I don't want to try too much and make the system unusable.

Its not hard to test this:

sudo service slim stop

(may not be slim.)

now try to 

startx 

or xinit -- :2

Which should get you default windowing environment
Once you have that all worked out you can remove gdm lightdm slim (or whichever(s) it is)

Perhaps this ~/.xinitrc may be of service::

https://git.devuan.org/PeteGozz/slim/bl … trc.sample

So until you are happy.

service slim restart

At some point you are ready to leave the Display Manager behind.
Or at least disable it as an interim move.

Next::

sudo update-rc.d slim disable

Finally: example only your Display Manager may be something else.

apt-get remove slim  

You _do_ read the list of _other_  things that will be removed and perhaps even installed!!

OK ? Confident ?

Then enter " y "

and _*welcome*_ to a simpler  life.

There may also be something useful here:
https://dev1galaxy.org/viewtopic.php?id=759