zserv-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/zserv-gfortran - from package: cernlib-packlib-gfortran

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

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