fatmen-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 CentOS possible default full paths are:

/usr/bin/fatmen-gfortran - from package: cernlib-packlib-gfortran

You may install the required package with command yum install {package}

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