I have a Goodman furnace gmnt080-4. The ignitor works but when it comes on, only one flame lights up, and then goes off.

You need to remove the burners & clean the small slots in the 'wings' of the burners. A thin piece of metal such as a feeler gauge works well. Then blow out the burners & slots with compressed air (the canned air you use on the computer keyboard will work too), put the burners back in & try it. You probably have rust in the slots.