Heder dropped out of the Will Ferrell-produced sitcom in March only days before production was to begin.
He allegedly left over "creative differences with the character", Heder's rep told Entertainment Weekly.
Gethard will take on the lead role of a computer geek who chases his dreams and moves back in with his parents.
Chris Parnell and Horatio Sanz have signed to co-star in the project, further produced by Chris Henchy and Adam McKay.
Gethard will also appear in Ferrell's upcoming film The Other Guys.