"My house had gravity-fed radiators for years. Boiler got hot, radiators got hot, very simple and it worked flawlessly. The radiators heated more evenly and almost as fast as with the modern circulator-pump system that replaced it. The new one may be more efficient, but that's due to boiler/oil-burner efficiency, not the circulation system. I think a gravity system requires greater water volume within the boiler than with forced circulation, but I could be wrong on that."
yes, gravity hot water is an option too.
but, single pipe steam is easier to design, one can shut of radiators not in use (to a point) without worry of pipes freezing, there is NO need for electricity ever and once the boiler is called for heat steam arrives very, very fast to the radiators, there's also only one pipe to each radiator, less time and $$ plumbing. as you may be able to tell i really, really like steam heat