gifdiff: 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/gifdiff - from package: gifsicle

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

On Debian possible default full paths are:

/usr/bin/gifdiff - from package: gifsicle

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

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