The Gettysburg Address was famous not because so many people were around to hear it, but mainly because of the words within the speech. The battle at Gettysburg was the turning point for the Union side to win. It was a victory for the Union side due to the large numbers of men who died on both side.
However, many more died on the Confederate side. When Lincoln gave his speech, only a few people were around to hear it. It was stated at the site of the battle and he talked about the soldiers who died there as well as what should happen with the war after this battle. The speech did not become very famous until well after it was said to encourage others through the war.