yindex-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/yindex-gfortran - from package: patchy-gfortran

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

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