VSZ also lists all libs, which while still useful doesn't take into account the sharing possible. If that's the only gtk2 app you run, then they are not shared, but otherwise they shouldn't count against the app.
What I use is "pmap -d $pid | tail -n1" (the private column). Others have also recommended the improved top apps.
For example, top VSZ lists 21mb for bash for me, while pmap says it's only 850kb. pmap -d without tail lists where the other parts are used, how much which lib takes etc.