Does the word "Our servant" in Allah's Almighty saying: "And if you are in doubt about what We have revealed to Our servant" mean that Muhammad (peace be upon him) is a servant of Allah and Jibreel, or of Allah alone? And is one who worships Jibreel alongside Allah, on the pretext that he is a messenger of Allah, considered a polytheist?

The plural pronoun in "Our servant" refers to Allah, Glorified be He, because it is a style from the styles of the Arabic language. Far be it from Allah that there should be any other true deity besides Him. And whoever worships Gabriel along with Allah, on the pretext that he is the Messenger of Allah, has indeed fallen into major polytheism (shirk al-akbar).