The 33-year-old actor popped the question to the costume designer "over the holidays," reported Us Weekly.
"They're very funny together and Thomas is always making the whole room laugh," an insider said.
Middleditch, best known for playing Richard Hendricks on the HBO hit "Silicon Valley" has been dating Gates for nearly three years.
Gates has worked in the costume department for movies including "The Twilight Saga: Breaking Dawn - Part 2", Lee Daniels' "The Butler" and "Dallas Buyers Club".