gxmessage: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/bin/gxmessage - from package: gxmessage

You may install the required package with command apt-get install {package}
Error in other languages:
gxmessage: bevel nie gevind nie
gxmessage: comandă negăsită
gxmessage: comando não encontrado
gxmessage: commande introuvable
gxmessage: command not found
gxmessage: käsku ei ole
gxmessage: không tìm thấy lệnh
gxmessage: komanda nerasta
gxmessage: Kommando nicht gefunden.
gxmessage: kommandot finns inte
gxmessage: komut yok
gxmessage: nie znaleziono polecenia
gxmessage: níor aimsíodh an t-ordú
gxmessage: no se encontró la orden
gxmessage: no s'ha trobat l'ordre
gxmessage: opdracht niet gevonden
gxmessage: parancs nem található
gxmessage: perintah tidak ditemukan
gxmessage: príkaz nenájdený
gxmessage: příkaz nenalezen
gxmessage: команда не найдена
gxmessage: командата не е открита
gxmessage: 命令找不到
gxmessage: コマンドが見つかりません