x86_64-linux-gnu-gcc-4.9: 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/x86_64-linux-gnu-gcc-4.9 - from package: gcc-4.9

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

Error in other languages:
x86_64-linux-gnu-gcc-4.9: bevel nie gevind nie
x86_64-linux-gnu-gcc-4.9: comandă negăsită
x86_64-linux-gnu-gcc-4.9: comando não encontrado
x86_64-linux-gnu-gcc-4.9: commande introuvable
x86_64-linux-gnu-gcc-4.9: command not found
x86_64-linux-gnu-gcc-4.9: käsku ei ole
x86_64-linux-gnu-gcc-4.9: không tìm thấy lệnh
x86_64-linux-gnu-gcc-4.9: komanda nerasta
x86_64-linux-gnu-gcc-4.9: Kommando nicht gefunden.
x86_64-linux-gnu-gcc-4.9: kommandot finns inte
x86_64-linux-gnu-gcc-4.9: komut yok
x86_64-linux-gnu-gcc-4.9: nie znaleziono polecenia
x86_64-linux-gnu-gcc-4.9: níor aimsíodh an t-ordú
x86_64-linux-gnu-gcc-4.9: no se encontró la orden
x86_64-linux-gnu-gcc-4.9: no s'ha trobat l'ordre
x86_64-linux-gnu-gcc-4.9: opdracht niet gevonden
x86_64-linux-gnu-gcc-4.9: parancs nem található
x86_64-linux-gnu-gcc-4.9: perintah tidak ditemukan
x86_64-linux-gnu-gcc-4.9: príkaz nenájdený
x86_64-linux-gnu-gcc-4.9: příkaz nenalezen
x86_64-linux-gnu-gcc-4.9: команда не найдена
x86_64-linux-gnu-gcc-4.9: командата не е открита
x86_64-linux-gnu-gcc-4.9: 命令找不到
x86_64-linux-gnu-gcc-4.9: コマンドが見つかりません