Citgo is a gas station. Rappers often used citgo in their songs to refer to weed. One of the type of gas one can buy from a gas station is diesel fuel, there's also a high quality weed strain call diesel, hence the reason why rappers use Citgo in their songs. For example, in Chief Keef's Citgo's song, he raps, "I’m smokin on the gas like Citgo." Chief Keef is basically saying that he's smoking on some really good weed.