gfortran-4.9: 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/gfortran-4.9 - from package: gfortran-4.9

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

Error in other languages:
gfortran-4.9: bevel nie gevind nie
gfortran-4.9: comandă negăsită
gfortran-4.9: comando não encontrado
gfortran-4.9: commande introuvable
gfortran-4.9: command not found
gfortran-4.9: käsku ei ole
gfortran-4.9: không tìm thấy lệnh
gfortran-4.9: komanda nerasta
gfortran-4.9: Kommando nicht gefunden.
gfortran-4.9: kommandot finns inte
gfortran-4.9: komut yok
gfortran-4.9: nie znaleziono polecenia
gfortran-4.9: níor aimsíodh an t-ordú
gfortran-4.9: no se encontró la orden
gfortran-4.9: no s'ha trobat l'ordre
gfortran-4.9: opdracht niet gevonden
gfortran-4.9: parancs nem található
gfortran-4.9: perintah tidak ditemukan
gfortran-4.9: príkaz nenájdený
gfortran-4.9: příkaz nenalezen
gfortran-4.9: команда не найдена
gfortran-4.9: командата не е открита
gfortran-4.9: 命令找不到
gfortran-4.9: コマンドが見つかりません