gdtopng: 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/gdtopng - from package: gd-progs

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

On Debian possible default full paths are:

/usr/bin/gdtopng - from package: libgd-tools

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