g3topbm: 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/g3topbm - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/g3topbm - from package: netpbm

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

Error in other languages:
g3topbm: bevel nie gevind nie
g3topbm: comandă negăsită
g3topbm: comando não encontrado
g3topbm: commande introuvable
g3topbm: command not found
g3topbm: käsku ei ole
g3topbm: không tìm thấy lệnh
g3topbm: komanda nerasta
g3topbm: Kommando nicht gefunden.
g3topbm: kommandot finns inte
g3topbm: komut yok
g3topbm: nie znaleziono polecenia
g3topbm: níor aimsíodh an t-ordú
g3topbm: no se encontró la orden
g3topbm: no s'ha trobat l'ordre
g3topbm: opdracht niet gevonden
g3topbm: parancs nem található
g3topbm: perintah tidak ditemukan
g3topbm: príkaz nenájdený
g3topbm: příkaz nenalezen
g3topbm: команда не найдена
g3topbm: командата не е открита
g3topbm: 命令找不到
g3topbm: コマンドが見つかりません