Qt as an ecosystem is fairly complete for general application development. It includes, among other things, concurrency primitives (QtConcurrent) that are trivial to use*, sockets, database support, serial port drivers and charting (qwt for example). Plus lots of useful tidbits I keep finding like accessing standard system temp directories, making temporary files and adding registry keys (or equivalent).

I've never used GTK in anger, but presumably there's some feature overlap. At times I've used Qt without any GUI components because it's so convenient (nicer than Boost for sure).

It also leverages native GUI frameworks so most applications will blend in with the platform you're writing for (last time I tried GTK this wasn't the default). It's easy to style with CSS if you need it.

Qt look and feel is much closer to native on Windows. Not 100%, but you need to know what to look for to notice it.

I would assume that the same is also the case on macOS.

From the perspective of a Windows user, GTK+ 3 does a less-good job of fitting in than Qt Widgets 5. Even with the win32 theme (GTK seems to use the Adwaita theme by default) GTK is missing shadows and the icon gutter in menus, and animations on push buttons and scrollbars. Qt Widgets 5 (with the Windows style, not the Fusion style) gets those right, and as a result, Qt apps look much more like native apps.

Gtk feels like it isn't actively developed anmore. Qt on the other hand feels like it's in active development.

THat's just an emotional "feeling". For hard facts, just compare the file picker dialogs.

GTK+3 fits well only into GNOME. If you want to make an app that works and behaves well under other DEs, Windows or macOS, then even GTK+2 would be a better choice. Basic stuff works, but as soon as you want to use something more platform specific, like notifications, event sounds, systray, platform native decorations etc. then GTK+3 is going to give you a hard time.