Have you tried adding it to the startup applications (search "startup applications")
If you have, try adding a .desktop file to
You can do this with the following command
sudo ln -s /usr/share/applications/guake.desktop /etc/xdg/autostart/
This is just an example, I don't know what the guake .desktop file is named but you can easily find out by having a look in the map: /usr/share/applications/
If by chance that doesn't work as well then there is one other solution you could try adding the following command to the startup applications
bash -c "sleep 10 && guake"
If it doesn't work try varying the sleep time because it depends on your system's speed but usually 10 seconds will work just fine.