x86_64-linux-gnu-gfortran: 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/x86_64-linux-gnu-gfortran - from package: gfortran

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

Error in other languages:
x86_64-linux-gnu-gfortran: bevel nie gevind nie
x86_64-linux-gnu-gfortran: comandă negăsită
x86_64-linux-gnu-gfortran: comando não encontrado
x86_64-linux-gnu-gfortran: commande introuvable
x86_64-linux-gnu-gfortran: command not found
x86_64-linux-gnu-gfortran: käsku ei ole
x86_64-linux-gnu-gfortran: không tìm thấy lệnh
x86_64-linux-gnu-gfortran: komanda nerasta
x86_64-linux-gnu-gfortran: Kommando nicht gefunden.
x86_64-linux-gnu-gfortran: kommandot finns inte
x86_64-linux-gnu-gfortran: komut yok
x86_64-linux-gnu-gfortran: nie znaleziono polecenia
x86_64-linux-gnu-gfortran: níor aimsíodh an t-ordú
x86_64-linux-gnu-gfortran: no se encontró la orden
x86_64-linux-gnu-gfortran: no s'ha trobat l'ordre
x86_64-linux-gnu-gfortran: opdracht niet gevonden
x86_64-linux-gnu-gfortran: parancs nem található
x86_64-linux-gnu-gfortran: perintah tidak ditemukan
x86_64-linux-gnu-gfortran: príkaz nenájdený
x86_64-linux-gnu-gfortran: příkaz nenalezen
x86_64-linux-gnu-gfortran: команда не найдена
x86_64-linux-gnu-gfortran: командата не е открита
x86_64-linux-gnu-gfortran: 命令找不到
x86_64-linux-gnu-gfortran: コマンドが見つかりません