How to change from GNOME to MATE

I'm new to the asking community altough I usually read the forum. My name is Maxi and I'm a PhD student from Argentina.

I just installed Debian testing (stretch) on my computer choosing both GNOME and MATE as possible desktop environments during the installation process. The thing is that GNOME shows up by default and I could not find a menu to choose MATE from. I remember using Ubuntu with Unity and KDE and being perfectly able to choose between the two each time I logged in. I tried several solutions found on the Inteet and they didn't work:

• I don't have a file called "~/.dmrc"
• I don't have a file called "lightdm-set-defaults" in /usr/lib
• My "xsessions" directory contains only "Default Xsession" and "MATE"
• I don't have a file called ".Xclients"; I created it with nothing but "mate-session" written in it, and nothing happened

Any insight will be very appreciated; I work with MATE in other 2 computers and really like it. Greetings, maxi.