xmessage is described as 'display a message or query in a window (X-based /bin/echo)' and is an app in the OS & Utilities category. There are more than 10 alternatives to xmessage for Linux, Windows, Mac, Python and Qt. The best alternative is Zenity, which is both free and Open Source. Other great apps like xmessage are Yad (Free, Open Source), KDialog (Free, Open Source), whiptail (Free, Open Source) and gxmessage (Free, Open Source).
Yad (yet another dialog) is a fork of Zenity with many improvements, such as custom buttons, additional dialogs, pop-up menu in notification icon and more. It's useful for creating quick and easy GUIs for shell and batch scripting.
My problem was that the KDialog code is still not ported to Qt5 and it depends on various Qt4/KDE4 components. The kdialog has big dependency sizes so we made an alternate dialogs like kdialog switches, options.