gemtopbm: command not found

Have You tried accessing the binary with the full path?

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/gemtopbm - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/gemtopbm - from package: netpbm

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