Sometimes it's useful to test things on older versions of GTK, specifically on older distributions. If you work at Red Hat, the main distribution you will use is most likely going to be Fedora. It's useful to know what versions had which version of GTK by default.
Determining versions for packages on Fedora
Visiting the site rpmfind will allow you to search for a package name. It also allows filtering by system (OS), and architecture.
For example, if you wanted to find which version of GTK3 was on Fedora 18, just type in GTK3 and filter by system "fedora". A list of entries will appear. A host of information including package name, distribution, and others will appear in a table. You can then scroll down to the Fedora 18 entries and find the package name. The package name will include the version number. In this case, GTK3.6 was used on Fedora 18 by default.