cmake-gui: 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/cmake-gui - from package: cmake-gui

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

On Debian possible default full paths are:

/usr/bin/cmake-gui - from package: cmake-qt-gui

You may install the required package with command apt-get install {package}

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