gbak: 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/gbak - from package: firebird

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

On Debian possible default full paths are:

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

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

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