gtkdialog: command not found

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 CentOS possible default full paths are:

/usr/bin/gtkdialog - from package: gtkdialog

You may install the required package with command yum install {package}

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