Answer: "Station" is a subject.
Explanation: In the noun + verb phrase "the station sold more gas," sold is used as a verb. A verb cannot be used behind a following verb. Additionally, "station" must function as the subject of the noun phrase "the station," as it is used with the determiner "the."
In short, because the station is being modified by the word "sold" and the content following, it must function as the subject.
