When you install a windowmanager by using make/not using apt-get install, it is possible gdm doesn't know there is a new window manager avaiable. You can help him by creating a new file .
Create the file by touch or the beloved vi.
sudo vi /etc/share/xsessions/notion.desktop
Input the following text.
[Desktop Entry] Encoding=UTF-8 Name=Notion Comment=This session logs you into Notion Exec=notion Icon= Type=Application