gunicorn: 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/gunicorn - from package: python-gunicorn

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

On Debian possible default full paths are:

/usr/bin/gunicorn - from package: gunicorn

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

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