gpasswd: 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/gpasswd - from package: shadow-utils

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

On Debian possible default full paths are:

/usr/bin/gpasswd - from package: passwd

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

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