for my intimate wedding, the left side (looking at the gazebo from the boardwalk) entrance was used as the "aisle"...I can imagine if you want it decorated, that is possible, but if your guests are seated in the center part, no one will really see the decorations, you know?
I don't see why you couldn't enter from the front portion and have it serve as the "aisle", and I imagine that for larger wedding parties, that could be how it works. For my wedding, it was just DH and me...so it didn't really matter which direction I approached from.
If you used the center aisle, I would think you could have some sort of aisle runner or flower petals or something, but since the seats for guests are just plastic, I'm not sure you could really do much with decorations there...
I'm sure that other brides have done more than I did (basic, no frills, off-the-rack intimate) and I'm sure they will share what they did with you...