Gandalf read runes, "five feet high the door anh three may walk abreast?" What would happen at the secret entrance if that was all they saw on the map?
If that were all to be seen on the map, then nothing would have happened. The fact of the matter is that the door was a magical, enchanted door, only to be opened with the special key carried by Thorin during sunset on Durin's Day. Elrond explains the significance of the moon-letters to Thorin while the company rests up in Rivendell:
"Stand by the grey stone when the thrush knocks," read Elrond, "and the setting sun with the last light of Durin's Day will shine upon the key-hole" (50).
Without the guidance of the moon-letters, Thorin and company might have continued sitting outside for a very long time, mostly because they would not have seen the key-hole appear. Even knowing the secret of the door, they would have missed it, except for Bilbo's sharp eyes and wit; after all, he is the one who hears the thrush's knocking at puts the clues together at the last minute, just barely in enough time for Thorin to secure the key-hole.
The details of the Lonely Mountain map in The Hobbit provides interesting clues for later plot development. Tolkien himself drew the original map which appears in most copies of The Hobbit. The map features the Lonely Mountain and surrounding areas. It also includes: a hand pointing to the hidden door, runes near the hand (these describe the size of the door), and another set of runes (moon letters) which tell them how to find the door.
If Gandalf and crew had only seen the first set of runes, they would have known the approximate location of the door, but would have been lost on how to make thes secret door known to them. The moon rune letters detail the specifics: the day, the time, the bird--the ultimate details that clue Bilbo into findingandopening the door to the Lonely Mountain.