Is Sikhi a Man made religion? Was Guru Nanak a Messenger of God or normal holy man?

It is claimed the Quran was revealed through Muhammad, the chosen Messenger and that the Angel Gabriel came to him, like the other Prophets to give the good news of their Prophet hood. Some fundamentalist Muslims claim Guru Nanak was a normal holy man who was revered by Hindus and Muslims alike, however not a Prophet or Messenger of God like Muhammad.

As for Gabriel informing the prophets, the Gurus were far above the angels. Baani tells us that angels are finite creatures and seek the dust of Gursikhs. It is said that Hazrat Muhammad became the Messenger of God at the age of 40. However, Guru Nanak Sahib Ji was born the embodiment of the Truth, the Image of God.
Guru Nanak Sahib Ji was informed of his duty by the Almighty Lord Himself, not by any angel:

ਪਉੜੀ ॥
ਹਉ ਢਾਢੀ ਹਰਿ ਪ੍ਰਭ ਖਸਮ ਕਾ ਹਰਿ ਕੈ ਦਰਿ ਆਇਆ ॥
ਹਰਿ ਅੰਦਰਿ ਸੁਣੀ ਪੂਕਾਰ ਢਾਢੀ ਮੁਖਿ ਲਾਇਆ ॥
ਹਰਿ ਪੁਛਿਆ ਢਾਢੀ ਸਦਿ ਕੈ ਕਿਤੁ ਅਰਥਿ ਤੂੰ ਆਇਆ ॥
ਨਿਤ ਦੇਵਹੁ ਦਾਨੁ ਦਇਆਲ ਪ੍ਰਭ ਹਰਿ ਨਾਮੁ ਧਿਆਇਆ ॥
ਹਰਿ ਦਾਤੈ ਹਰਿ ਨਾਮੁ ਜਪਾਇਆ ਨਾਨਕੁ ਪੈਨਾਇਆ ॥੨੧॥੧॥ ਸੁਧੁ
“Pauree: I was free of work, when Vaheguru took me into His service and gave the (true) work of being minstrel. Right from the start, Vaheguru gave me His Hukam (Order) to sing His Praises day and night. Vaheguru, my Lord and Master summoned me, His minstrel, to the True Mansion of His Presence. (Vaheguru,) the Image of True Praise and Glory, gave me the Siropaao, robe of honour. The spiritual-life giving Name, Amrit Naam, the True Name, which gives eternal spiritual life, has become my food (for my soul’s support). Those who follow the Guru’s Teachings, who eat this food (of ‘Amrit Naam’) with satisfaction, obtain peace. His minstrel, (Guru Nanak Sahib jee), spreads His Glory, singing and vibrating the Word of His Shabad (meaning, enjoying the ecstasy of Naam). O Nanak! Praising the True Lord, one obtains the Perfect Lord. ||27||Shudh||”
(Ang 150)