gex: 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/gex - from package: vim-X11

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

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