cmake3-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/cmake3-gui - from package: cmake3-gui

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

Error in other languages:
cmake3-gui: bevel nie gevind nie
cmake3-gui: comandă negăsită
cmake3-gui: comando não encontrado
cmake3-gui: commande introuvable
cmake3-gui: command not found
cmake3-gui: käsku ei ole
cmake3-gui: không tìm thấy lệnh
cmake3-gui: komanda nerasta
cmake3-gui: Kommando nicht gefunden.
cmake3-gui: kommandot finns inte
cmake3-gui: komut yok
cmake3-gui: nie znaleziono polecenia
cmake3-gui: níor aimsíodh an t-ordú
cmake3-gui: no se encontró la orden
cmake3-gui: no s'ha trobat l'ordre
cmake3-gui: opdracht niet gevonden
cmake3-gui: parancs nem található
cmake3-gui: perintah tidak ditemukan
cmake3-gui: príkaz nenájdený
cmake3-gui: příkaz nenalezen
cmake3-gui: команда не найдена
cmake3-gui: командата не е открита
cmake3-gui: 命令找不到
cmake3-gui: コマンドが見つかりません