gcal: command not found

Have You tried accessing the binary with the full path?

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/gcal - from package: gcal

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