StackOverflow is becoming the go-to place for technical Q&A. Is it becoming an indicator of future technology trends?
No, of course not.
The only thing this proves is that more people who use Qt ask/answer questions on Stack Overflow than do people who use GTK. It doesn't prove that no one uses GTK anymore or that Qt is more popular.
This is as silly as the people who ask "Why are there so many questions about C# on Stack Overflow?", and then go on to attribute it to either a Microsoft-led conspiracy or the fact that since people have so many questions about C# that it must be harder to use than any other language. It should really go without saying that both are quite obviously wrong.
If there's truth to this conjecture then perhaps GTK will be generally legacy code a generation from now?
This is basically nonsense to my ears, too. First off, we don't really know what "a generation" is when it comes to technology, much less programming languages. Second, as you mention, GTK will still have important applications, like for applications programmed in C (rather than C++, as required by Qt). Third, the designator of "legacy code" is basically worthless. Who decides when something is "legacy"? Is it the people who are still working on those projects and wish they weren't? Is it the people who are pushing the latest and greatest technology that replaces it? Is it someone on the Internet with a popular blog?
There's no brightline for when to ascribe "legacy" status to a particular technology, and doing so doesn't make much sense, either. If GTK still works for you, keep using it. If it doesn't, it might be time to investigate alternative solutions. Drawing conclusions like this "a generation" in the future isn't particularly worthwhile anyway, as there's no reason to suspect that another company won't release the next greatest GUI framework for C++ that overtakes Qt and renders it "obsolete" or "legacy" or whatever.