I don't understand why you are discussing the qt library when the original question was about GTK. GTK3 development in the last year was awesome! Gnome, elementary, Mate and Cinnamon and last but not least XFCE are only some of the desktop environments that are using GTK3. So how can you say qt is more "serious"?
On Linux it would be a breath of death for Lazarus if it does not support GTK3 in the next years. At least for me, it would be more interesting to be able to use GTK3 than Pascal so I would move to C, Vala, D, Python, Ruby, Lua or whatever, which, by the way, in contrast to FreePascal have excellent GTK3 bindings since years!
But instead of ranting, I would like to change the situation and again would like to call all developers who are unsatisfied with the current situation to join and to coordinate the development on the GTK3 bindings!
Best regards,
Marc