arm-none-eabi-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 Debian possible default full paths are:

/usr/bin/arm-none-eabi-gdb - from package: gdb-arm-none-eabi

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

Error in other languages:
arm-none-eabi-gdb: bevel nie gevind nie
arm-none-eabi-gdb: comandă negăsită
arm-none-eabi-gdb: comando não encontrado
arm-none-eabi-gdb: commande introuvable
arm-none-eabi-gdb: command not found
arm-none-eabi-gdb: käsku ei ole
arm-none-eabi-gdb: không tìm thấy lệnh
arm-none-eabi-gdb: komanda nerasta
arm-none-eabi-gdb: Kommando nicht gefunden.
arm-none-eabi-gdb: kommandot finns inte
arm-none-eabi-gdb: komut yok
arm-none-eabi-gdb: nie znaleziono polecenia
arm-none-eabi-gdb: níor aimsíodh an t-ordú
arm-none-eabi-gdb: no se encontró la orden
arm-none-eabi-gdb: no s'ha trobat l'ordre
arm-none-eabi-gdb: opdracht niet gevonden
arm-none-eabi-gdb: parancs nem található
arm-none-eabi-gdb: perintah tidak ditemukan
arm-none-eabi-gdb: príkaz nenájdený
arm-none-eabi-gdb: příkaz nenalezen
arm-none-eabi-gdb: команда не найдена
arm-none-eabi-gdb: командата не е открита
arm-none-eabi-gdb: 命令找不到
arm-none-eabi-gdb: コマンドが見つかりません