gnatclean: 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/gnatclean - from package: gcc-gnat

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

On Debian possible default full paths are:

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

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

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