Only on Windows, as it may be the only semi-reliable GL implementation available there.
No, there are no plans, and nobody is working on that, to the best of the GTK team’s knowledge.
Targeting Android is not trivial, especially for GUI applications. It’s even worse for iOS. I would never recommend using anything that tried to reimplement Android’s or iOS’s UI toolkits and drawing directly on a windowing system surface; my suggested approach would be something like Xamarin’s, with a generic API that gets compiled for each platform and turned into platform-native widgets. Alternatively, writing your business logic without any toolkit, and then having different UIs for different platforms.
No, it really would not.
Established mobile platforms come with their own platform toolkits and libraries for a reason, and it takes a lot less to retrain (or hire) people on different platforms than it takes to write a toolkit that abstracts them all away, poorly.
I know that some people use Qt for that, but it’s pretty clear that Qt is not making any inroads in those areas, and it’s mostly niche free software apps that use it.