gif2webp: 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/gif2webp - from package: libwebp-tools

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

On Debian possible default full paths are:

/usr/bin/gif2webp - from package: webp

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

Error in other languages:
gif2webp: bevel nie gevind nie
gif2webp: comandă negăsită
gif2webp: comando não encontrado
gif2webp: commande introuvable
gif2webp: command not found
gif2webp: käsku ei ole
gif2webp: không tìm thấy lệnh
gif2webp: komanda nerasta
gif2webp: Kommando nicht gefunden.
gif2webp: kommandot finns inte
gif2webp: komut yok
gif2webp: nie znaleziono polecenia
gif2webp: níor aimsíodh an t-ordú
gif2webp: no se encontró la orden
gif2webp: no s'ha trobat l'ordre
gif2webp: opdracht niet gevonden
gif2webp: parancs nem található
gif2webp: perintah tidak ditemukan
gif2webp: príkaz nenájdený
gif2webp: příkaz nenalezen
gif2webp: команда не найдена
gif2webp: командата не е открита
gif2webp: 命令找不到
gif2webp: コマンドが見つかりません