g_x2top: 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_x2top - from package: gromacs

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

On Debian possible default full paths are:

/usr/bin/g_x2top - from package: gromacs

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

Error in other languages:
g_x2top: bevel nie gevind nie
g_x2top: comandă negăsită
g_x2top: comando não encontrado
g_x2top: commande introuvable
g_x2top: command not found
g_x2top: käsku ei ole
g_x2top: không tìm thấy lệnh
g_x2top: komanda nerasta
g_x2top: Kommando nicht gefunden.
g_x2top: kommandot finns inte
g_x2top: komut yok
g_x2top: nie znaleziono polecenia
g_x2top: níor aimsíodh an t-ordú
g_x2top: no se encontró la orden
g_x2top: no s'ha trobat l'ordre
g_x2top: opdracht niet gevonden
g_x2top: parancs nem található
g_x2top: perintah tidak ditemukan
g_x2top: príkaz nenájdený
g_x2top: příkaz nenalezen
g_x2top: команда не найдена
g_x2top: командата не е открита
g_x2top: 命令找不到
g_x2top: コマンドが見つかりません