gif2rgb: 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/gif2rgb - from package: giflib-utils

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

On Debian possible default full paths are:

/usr/bin/gif2rgb - from package: giflib-tools

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

Error in other languages:
gif2rgb: bevel nie gevind nie
gif2rgb: comandă negăsită
gif2rgb: comando não encontrado
gif2rgb: commande introuvable
gif2rgb: command not found
gif2rgb: käsku ei ole
gif2rgb: không tìm thấy lệnh
gif2rgb: komanda nerasta
gif2rgb: Kommando nicht gefunden.
gif2rgb: kommandot finns inte
gif2rgb: komut yok
gif2rgb: nie znaleziono polecenia
gif2rgb: níor aimsíodh an t-ordú
gif2rgb: no se encontró la orden
gif2rgb: no s'ha trobat l'ordre
gif2rgb: opdracht niet gevonden
gif2rgb: parancs nem található
gif2rgb: perintah tidak ditemukan
gif2rgb: príkaz nenájdený
gif2rgb: příkaz nenalezen
gif2rgb: команда не найдена
gif2rgb: командата не е открита
gif2rgb: 命令找不到
gif2rgb: コマンドが見つかりません