Major versions of Gtk are, for all intents and purposes, different toolkits entirely. They are always parallel-installable, they don't conflict with each other.
Well, except for the part that development stops in previous versions and they do not get any real updates while they "hog" the "Gtk" name so any forks that may want to continue development in a backwards compatible way as if the incompatible change never happened cant really be called "Gtk" without being misleading.