gmail-notify: 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 Debian possible default full paths are:

/usr/bin/gmail-notify - from package: gmail-notify

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

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