xgamma: 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/xgamma - from package: xorg-x11-server-utils

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

On Debian possible default full paths are:

/usr/bin/xgamma - from package: x11-xserver-utils

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

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