gxmessage is described as 'a GTK-based xmessage clone' and is a Shell in the OS & Utilities category. There are more than 10 alternatives to gxmessage for Linux, Windows and Mac. The best alternative is Zenity, which is both free and Open Source. Other great apps like gxmessage are Yad (Free, Open Source), KDialog (Free, Open Source), whiptail (Free, Open Source) and MessagePop (Free).
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.
Xdialog is designed to be a drop in replacement for the "dialog" or "cdialog" programs. It converts any terminal based program into a program with an X-windows interface. The dialogs are easier to see and use while adding even more functionalities (e.g.