g.gui: 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/lib64/grass-6.4.4/bin/g.gui - from package: grass

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

On Debian possible default full paths are:

/usr/lib/grass64/bin/g.gui - from package: grass-core

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

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