To be honest, I never heard of "gravity" bleeding brakes until I got on the forums on the net. My personal background is that of having vocational auto mechanics training at the high school and trade school level, 10+ years as a working mechanic in dealership, independent and tire shops. 13 years as a vocational auto mechanics teacher at the high school level. Gravity bleeding seems to be a "hobbyist thing that may or may not work due to the application.
Due to not having a pressure bleeder at home. I usually use my mity-vac vacuum bleeder or have someone pump the brakes.
Doing the pump the brakes thing still seems to be the fastest and most positive way to get a good pedal if you don't have a pressure bleeder.
A couple of notes on doing the pump the brake bleeding thing.
The person pumping the brakes has to pump "very slowly" deliberately so as I have seen way too many helpers want to pump the pedal too fast.
Secondly, they have to hold the pedal down while you open the bleeder and keep it down until you close the bleeder and tell them "ok pump it again".
Last, make sure you never run out of fluid in the mc no matter what method you use or you get to start over.
There is an online article about making your own pressure bleeder out of a hardware store half gallon weed sprayer.
You would have to adapt your own cap for the master cylinder you have but putting a fitting on a used cap and trimming the gasket shouldn't be a big issue. OOPs, the article
http://www.bmw-m.net/TechProc/bleeder.htm