apt-cache search "for gnome" | wc -l -
There you have the total of packages which include the string "for Gnome" in their description. You could reduce the list futher by adding a filter for "graphical" or "gui", like this:
apt-cache search "for gnome" graphical | wc -l -
If you want to save the list into a file for further reference, just type:
apt-cache search "for gnome" > file.txt
Of course, the .txt extension is not needed by Linux desktop to know how to handle it.