Sikhism is unusual among world religions in that the founder's compositions survive in a scripture compiled by his own successors, in his own language, with a manuscript tradition that can be traced. This makes the distinction between what he said and what was said about him unusually clean.
The Guru Granth Sahib contains 974 hymns attributed to Nanak, including the Japji Sahib that opens it and the Mul Mantar that opens that. The compilation was made by Guru Arjan, the fifth Guru, in 1604, and there is a manuscript tradition running to the Kartarpur volume. Pashaura Singh and others have worked on the textual history in detail, and the process was one of deliberate editorial selection, which is itself informative.
The biographical material is a different order of evidence. The janamsakhis, birth stories, are hagiographies compiled from the seventeenth century onward, well after his death in 1539, and they are full of miracles, long journeys, and encounters with named figures. Several traditions of janamsakhi exist and they disagree with each other. The claim that Nanak travelled to Mecca, Baghdad, Sri Lanka and Tibet comes from these, not from his own writings.
The best-known single story, that he disappeared into a river for three days and emerged saying there is no Hindu, there is no Muslim, is janamsakhi material. It captures something the hymns genuinely support, but it is not a record of an event.
Sikh scholarship has debated the historicity of the janamsakhis openly and sometimes bitterly, and it is an internal argument as much as an external one.