According to the site's source, Benicio Del Toro is playing The Collector, one of the Elders of the Universe and a powerful being who prophesies the rise of Thanos, creating a museum to "collect" and store life forms to save them from extinction at the hands of the Mad Titan. Pace's character is said to be that of Ronan the Accuser, a high-ranking member of the Kree Empire (and son of Thanos, in the Ultimate universe), while Gillan is reportedly Nebula, a space pirate who crosses Thanos by stealing one of his ships and claiming to be his granddaughter.
With the San Diego Comic-Con just around the corner, we should probably expect an official announcement on the cast of Guardians of the Galaxy as part of Marvel's Hall H presentation, which will also showcase the upcoming solo sequels Thor: The Dark World and Captain America: The Winter Soldier.
Guardians of the Galaxy is set for release on August 1st 2014, with James Gunn (Super) directing a cast that includes Chris Pratt (Parks and Recreation) as Peter Quill/Star-Lord, Zoe Saldana (Avatar) as Gamora, Dave Bautista (Riddick) as Drax the Destroyer, Michael Rooker (The Walking Dead) as Yondu, John C. Reilly (Step Brothers) as Rhomman Dey and Glenn Close (Damages) as the leader of the Nova Corps.