gbirthday: 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/gbirthday - from package: gbirthday

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

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