How to unlock lower levels in portal 2?

I am playing this game from last few days and the thing here is that the lower levels of the game are locked I mean when tried to enter the lower levels of Apperture and found the giant door blocking me off and the access was not provided to me I don’t know why this is happening to me I guess the levels are unlocked but then I am not able to find a perfect way to get rid of them that is I am unaware of how to unlock the same. So please help me in this one and let me know what I can do to unlock the same.

The giant door about which you are talking is probably protecting the little push door and that is the reason why you are not allowed to pass from that particular area or level. Now you have to gain as much gels as possible so that you will be able to enter the door which you are talking. I am sure that once the doors are cleared with the help of gels then you will definitely able to get rid of the problem.

The basic thing that I would like to share here is that if you are playing the game and that to a steam one make sure that you are clearing all the levels initially I men if the lower levels are unlock then I want you to just make sure that you have cleared all the starting phases. I hope in this case the lower levels that is the Apperture is unlocked because you haven’t cleared the same. Now if you need to unlock these levels then please try and do as I say.

Well to clear these levels you have to use the gravity to its best to give you an extra boost. Now shoot the portal which will be on the roof and also there will be one under you so get that one also after hitting it you will be continuously falling so keep one thing in mind that if you really want to go in the lower area then you need to get all the gels with you and then kill the other abstracting portals. I hope that you are getting my point.