When comparing GTK+ vs Xojo, the Slant community recommends GTK+ for most people. In the question“What are the best cross-platform GUI toolkits?” GTK+ is ranked 8th while Xojo is ranked 21st. The most important reason people chose GTK+ is:
Ranked in these QuestionsQuestion Ranking
Pro Language bindings
Pro Nicer user interface when using PyGi
Creating cross platform enterprise apps with PyGi and with the help of Gtk Themes makes them look great.
Pro Easy to use
Easy to make compiled native apps.
Con Terrible Integration
Looks and feels like an alien on all other platforms than GNOME.
Con Subscription-based/Not free
Using this product you're betting the company will still be around. If they get bought out by an enterprise that wants to use the tech internally, all they need to do is stop offering subscriptions and suddenly you're scrambling to rebuild your entire app on other technology.
Con No Android support
They plan to eventually support Android, but for now, they only support iOS.