It happened around 11 p.m. Wednesday at the Chevron station at Mays Avenue and Magnolia Street, just off the 22 Freeway.
Store surveillance video caught the suspect entering the store wearing a red baseball cap and a bandana over his face.
Police say he was carrying a large, semi-automatic handgun.
The suspect pointed the weapon at the clerk, handed him a bag and ordered him to empty the cash register.
The robber got away with about $2,000, according to police.
He is described as a black male about 5’11” tall and 250 pounds.
He was wearing a blue, short-sleeved, button-up plaid shirt over a long-sleeved black sweatshirt or T-shirt, jeans and white gardening-type gloves.
Anyone with information is asked to call Garden Grove police.