Allah SWT blessed His beloved Prophet with a lot of knowledge. However, the word "Nabi" means the one who bring news or information. "Nabiul-Ghaib" will mean the informer of what is hidden.
Example: In surat Al-Mulk, Allah SWT says, 25. They ask: When will this promise (of resurrection) be (fulfilled)? - If ye are telling the truth.
26. Say: "As to the knowledge of the time, it is with Allah alone: I am (sent) only to warn plainly in public."
