mpiexec.gforker: 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/mpiexec.gforker - from package: mpich2
/usr/lib64/mpich/bin/mpiexec.gforker - from package: mpich
/usr/lib/mpich/bin/mpiexec.gforker - from package: mpich

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

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