Revenge for Jolly! (2012) [Tina]: Shot twice off-camera in her chest (with exit wound through her back, blood splattered on the wall behind her) by Brian Petsos, after Oscar Isaac fires a warning shot into the wall beside Gillian (and Brian assumes that Oscar actually meant to kill her but missed).
Seeking a Friend for the End of the World (2012) [Waitress]: Killed (off-screen), along with everybody else on the planet, when an asteroid collides with Earth creating the solar induced fire storm that was destroying the Earth's surface. (Thanks to Tommy and Tim)
The Lookalike (2014) [Lacey Fitzgerald/Sadie Hill]: In a dual role, Gillian plays Sadie, the love interest of mob kingpin John Savage, who is accidentally killed by a falling chandelier while talking to low-level mobsters John Corbett and Steven Bauer. Lacey is Sadie's exact double and is hired to play her by the two crooks in order to fool their boss. Lacey survives the movie.
Visions (2015) [Sadie]: Throat slashed by Isla Fisher, who used a piece of her bed's headboard as a weapon.
Community: Horror Fiction in Seven Spooky Steps (2011) [Britta Perry]: Dies in several of the story-within-the-story sequences: (1) Stabbed in the chest with a hook by Ken Jeong. (2) Drained of blood by Joel McHale. (3) Commits suicide by stabbing herself. (4) Slashed to death with a chainsaw by Jim Rash. She survives the episode in reality, obviously. (All played for comic effect.) (Thanks to Tommy)
The Venture Bros.: Venture Libre (2013; animated) [Marsha Blackwood]: Killed in an explosion by a booby-trapped iPad; she is later brought back to life as a Frankenstein-like creature by Dr. Venture (voiced by James Urbaniak).