His other films include Dear White People, American Sniper (both 2014), Dinner In America (2020), The Passenger, and Strange Darling (both 2023).

He also guest starred on Cold Case as a teenaged gunman in the episode "Rampage", and appeared in the independent film Sublime.

In 2007, he guest starred in an episode of Law & Order: Special Victims Unit titled "Impulsive" as a high school student who accuses his teacher (played by Melissa Joan Hart) of statutory rape.

[8] In 2013, Gallner co-founded the independent production company Minutehand Pictures along with Oliver Thompson and Bay Dariz.

The trio produced the feature film Welcome to Happiness, in which Gallner portrays Woody Ward, a children's book author with a secret door in his closet.

[9] In 2014, Gallner played the role of Kurt, the antagonist to lead character Sam, in the independent film Dear White People.

He starred in the critically acclaimed comedy Dinner in America, which premiered at Sundance in 2020 and earned him praise for his performance opposite Emily Skeggs.