rgb2gif: 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/rgb2gif - from package: giflib-utils

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

On Debian possible default full paths are:

/usr/bin/rgb2gif - from package: giflib-tools

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

Error in other languages:
rgb2gif: bevel nie gevind nie
rgb2gif: comandă negăsită
rgb2gif: comando não encontrado
rgb2gif: commande introuvable
rgb2gif: command not found
rgb2gif: käsku ei ole
rgb2gif: không tìm thấy lệnh
rgb2gif: komanda nerasta
rgb2gif: Kommando nicht gefunden.
rgb2gif: kommandot finns inte
rgb2gif: komut yok
rgb2gif: nie znaleziono polecenia
rgb2gif: níor aimsíodh an t-ordú
rgb2gif: no se encontró la orden
rgb2gif: no s'ha trobat l'ordre
rgb2gif: opdracht niet gevonden
rgb2gif: parancs nem található
rgb2gif: perintah tidak ditemukan
rgb2gif: príkaz nenájdený
rgb2gif: příkaz nenalezen
rgb2gif: команда не найдена
rgb2gif: командата не е открита
rgb2gif: 命令找不到
rgb2gif: コマンドが見つかりません