gfix: 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/gfix - from package: firebird

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

On Debian possible default full paths are:

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

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

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