How to get alt+right mouse to resize windows again!

In more recent gnome versions (e.g., gnome-shell), you need to use this instead:

gsettings set org.gnome.desktop.wm.preferences resize-with-right-button true

Gnome defaults to using the Super ("Windows") key for window actions, so the above alone will enable moving (super-leftdrag) and resizing (super-rightdrag). To use the Alt key instead of the Super key do:

gsettings set org.gnome.desktop.wm.preferences mouse-button-modifier '<Alt>'

(note that using the Alt key for window operations will interfere with some apps, like Inkscape, that use alt-click and alt-drag for app related actions)

If you install GNOME Tweaks, there's an option to "Resize with secondary click". Checking it will enable Super+RMB to resize windows.

Additionally to using gsettings and instead of using the deprecated gconf-editor, you can install the newer dconf-editor and find and change relevant keys there. Searching "resize with" will find its key regardless of used WM.