@codebrainz, @elextr, If GTK's widgets window have a native Handle, then it is possible to delegate the embedding job to ConEmu himself. By the [docs](https://conemu.github.io/en/InsideParent.html):
The only thing you shall implement or provide is some child window, which ‘HWND’ you pass as ConEmu.exe switch -insidewnd 0xHWND when you run the terminal. HWND is hexadecimal window descriptor.