gdef: 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/gdef - from package: firebird

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

On Debian possible default full paths are:

/usr/bin/gdef - from package: firebird2.5-classic-common
/usr/bin/gdef - from package: firebird2.5-super

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

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