g_xpm2ps: 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/g_xpm2ps - from package: gromacs

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

Error in other languages:
g_xpm2ps: bevel nie gevind nie
g_xpm2ps: comandă negăsită
g_xpm2ps: comando não encontrado
g_xpm2ps: commande introuvable
g_xpm2ps: command not found
g_xpm2ps: käsku ei ole
g_xpm2ps: không tìm thấy lệnh
g_xpm2ps: komanda nerasta
g_xpm2ps: Kommando nicht gefunden.
g_xpm2ps: kommandot finns inte
g_xpm2ps: komut yok
g_xpm2ps: nie znaleziono polecenia
g_xpm2ps: níor aimsíodh an t-ordú
g_xpm2ps: no se encontró la orden
g_xpm2ps: no s'ha trobat l'ordre
g_xpm2ps: opdracht niet gevonden
g_xpm2ps: parancs nem található
g_xpm2ps: perintah tidak ditemukan
g_xpm2ps: príkaz nenájdený
g_xpm2ps: příkaz nenalezen
g_xpm2ps: команда не найдена
g_xpm2ps: командата не е открита
g_xpm2ps: 命令找不到
g_xpm2ps: コマンドが見つかりません