Gill Sans digital font, as digitized by Monotype Corporation, is owned and copyrighted by them.
A typeface itself cannot really be patented nor copyrighted, only trademarked. An artist could therefore digitize the font himself and either copyright his digitization, or release it into PD or under SIL OFL licence and Monotype can do nothing about it, as long as the artist doesn't use the Gill Sans name.
You see, Monotype has trademarked Gill Sans at some point thru a trademark transfer from Eric Gill or some heir of his (AFAIK). They had to, since otherwise either the trademark was in public domain already, or Monotype breached the trademark of the original trademark owner.
However the original trademark by Eric Gill has expired into public domain and, legally, even in the US, transfer of trademark to another party should not extend it's life, which is why I believe trademark information Monotype is giving is tad misleading and their actual legal power to litigate on this particular trademark is limited.
Still, as most people know, it's safer not to mess with companies with huge legal departments.