gifsicle: 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/gifsicle - from package: gifsicle

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

On Debian possible default full paths are:

/usr/bin/gifsicle - from package: gifsicle

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

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