Here a citation from Wikipedia, explaining the meaning of this sealed gate:
"In Jewish tradition, this is the gate through which the Messiah will enter Jerusalem. Ottoman Sultan Suleiman I sealed off the Golden Gate in 1541 to prevent the Messiah's entrance. The Muslims also built a cemetery in front of the gate, in the belief that the precursor to the Messiah, Elijah, would not be able to pass through the Golden Gate and thus the Messiah would not come. This belief was based upon two premises. First, according to Islamic teaching Elijah is a descendant of Aaron making him a priest or kohen. Secondly, that Jewish priests are not permitted to enter a cemetery. This second premise is not wholly correct because a kohen is permitted to enter a cemetery in which primarily non-Jews are buried such as the one outside the Golden Gate."