Usually GDM tries to calculate the scaling factor by itself, but it doesn't always work well.
Open a schema from glib-2:
sudo vim /usr/share/glib-2.0/schemas/org.gnome.desktop.interface.gschema.xml
Find the following segment of code:
<key name="scaling-factor" type="u">
<default>0</default>
and replace the value of <default>
with 2 (or 3, or 4 - whatever is to your liking). 0 will make GDM try to set scaling-factor
automatically.
Save and quit the file. Then run the following command:
sudo glib-compile-schemas /usr/share/glib-2.0/schemas
Done! Now log out and verify that the scaling has changed.