You can't miss this, so long as you play through the game all the way.
The gravity machine levels are levels 171-180. These levels are kind of tricky and often times require a lot of good timing. Often times in these levels the switching of gravity will mess up the level completely seeing as how each section of the game adds a new feature and presents you with features from previous levels. Gravity can go from right side up to upside down, or left to right, or in one instance every direction seemingly.
The developer has a walkthrough of the game, its the mobile version but applies to the PC version as well.