g_rdf: 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_rdf - from package: gromacs

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

On Debian possible default full paths are:

/usr/bin/g_rdf - from package: gromacs

You may install the required package with command apt-get install {package}

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