Discones have 0dB gain. How much additional signal you may get by putting an antenna outside in an elevated location depends on the materials in your building, your indoor antenna, the coax you use, the topography in your area, and various other factors.
If you're on top of a mountain in a small wooden cabin, an outdoor antenna won't help all that much; you'd have a fairly clear line of sight either way, and the outdoor antenna may only get you an additional 3-5dB of signal.
But if you're in a building with aluminum siding in a small valley, the outdoor antenna may give you an additional 30dB of signal, if it offers a clear line of sight to the transmitter.
You'll probably find yourself between these extremes.