Hi, Peter, I've dig too. But not found anything. I don't understand well GTK2Widgets. And i can't test. The better will be ask in the Unix forum here. I'll do some deep search.
In Waiting, perhaps, try changing vGTKWidget^.window to vGTKWidget^.parent^.Window
One thing else you can try to add a TPanel on the tab and TGLViewer on this TPanel to see.
jérôme.