Recently Xojo 2017r2 was released which uses GTK in version 3.0 on Linux. Previous versions used GTK 2.
You can load a copy on my plugin download page.
This plugin is build with my plugin build system, so it has MBS in the name and includes libraries for Mac, Windows and Linux including ARM, so you can use it in all projects. If you find a bug related to the building it, please tell me.
A speciality of MBS Plugins with GTK is, that we built plugins to link dynamically to GTK and use either GTK 2 or GTK 3 depending on the Xojo version. This should help to have just one set of plugins to work with Xojo 2013 up to 2020.