gnathtml: 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 Debian possible default full paths are:

/usr/bin/gnathtml - from package: gnat-4.9

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

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