gdb: 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/gdb - from package: gdb

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

On Debian possible default full paths are:

/usr/bin/gdb - from package: gdb-minimal
/usr/bin/gdb - from package: gdb

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

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