People believed that Muhammad was a prophet primarily because they believed that he received messages from Allah (God). According to Islamic tradition, Muhammad received revelations from Allah through the angel Gabriel over a period of approximately 23 years. These messages were later compiled into the Quran, which is considered by Muslims to be the literal word of God. His ability to convey these messages, his ethical teachings, and his leadership in the early Muslim community contributed to the belief in his prophethood.
The other options you provided do not accurately reflect the beliefs held about Muhammad. For instance, although Gabriel is significant as the messenger of revelation, Muhammad is not believed to be Gabriel himself. Additionally, while he was recognized as a fair leader and played a vital role in establishing the Muslim community, the foundational belief in his prophethood comes from the revelations he received rather than from founding Mecca or other factors.