gnat: 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/gnat - from package: gcc-gnat

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

On Debian possible default full paths are:

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

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

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