g_pdb2gmx: 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_pdb2gmx - from package: gromacs

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

Error in other languages:
g_pdb2gmx: bevel nie gevind nie
g_pdb2gmx: comandă negăsită
g_pdb2gmx: comando não encontrado
g_pdb2gmx: commande introuvable
g_pdb2gmx: command not found
g_pdb2gmx: käsku ei ole
g_pdb2gmx: không tìm thấy lệnh
g_pdb2gmx: komanda nerasta
g_pdb2gmx: Kommando nicht gefunden.
g_pdb2gmx: kommandot finns inte
g_pdb2gmx: komut yok
g_pdb2gmx: nie znaleziono polecenia
g_pdb2gmx: níor aimsíodh an t-ordú
g_pdb2gmx: no se encontró la orden
g_pdb2gmx: no s'ha trobat l'ordre
g_pdb2gmx: opdracht niet gevonden
g_pdb2gmx: parancs nem található
g_pdb2gmx: perintah tidak ditemukan
g_pdb2gmx: príkaz nenájdený
g_pdb2gmx: příkaz nenalezen
g_pdb2gmx: команда не найдена
g_pdb2gmx: командата не е открита
g_pdb2gmx: 命令找不到
g_pdb2gmx: コマンドが見つかりません