rainbow-gc: 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/sbin/rainbow-gc - from package: rainbow

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

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