I imagine there may have been other way's to reach the front door. Remember that Aragorn was rather anxious to reach the safety of Lothlorien before the orcs of Khazad Dum were able to regroup and chase after the Fellowship.
All Gollum had to do was follow the pursuing orcs, through any other existing passageways to the door. Or wait until the orcs had devised a means of crossing over the broken section of the bridge. Then again, perhaps the orcs had added other exits (and again Gollum only need follow behind the orcs).
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