gnatxref: 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/gnatxref - from package: gcc-gnat

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

On Debian possible default full paths are:

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

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

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