It seems like I have only two rooms left (got the red gun), Laying The Foundation (top left in the map; I know what the problem is, there) and A Jump Too Far (in the middle). On other screenshots I have seen at least three more rooms in the section marked red, but I can't find any route I haven't taken a dozen times. Is it something incredibly obvious?! (or is this through the door marked The End? It's still blocked…)
You have to use the red gun to unblock and then to go through the door marked "The End".
Look at the floor, and pay attention to details, like a small gap.
I would like to point out that it is perfectly possible to get through here with only the yellow gun. I wouldn't recommend it though as it is far easier to complete with the red gun. Just use the gap in the floor and yellow gun to drag the blocks up/use the red gun to grow blocks destroying the blocks preventing you from proceeding.
