Reach the following population level for each plane to unlock the corresponding gift structures. These structures can only be built once, and cost nothing:

POPULATION LEVEL - GIFT STRUCTURE
50,000 - The Vista Enhancement Doohicky and The Ugliness Engine
500,000 - The Audio Improving Embophone and The Crinkly Cacophony Contrivance
5 million - The Heaven Scent Atomizer and The Flabbergasting Flatulence Ol-Factory
50 million - The Creme Candy Castle and The Wellspring of unsavoriness
500 million - The Floofy Comfort Dispenser and The Tactile Degradation Gizmo
1 billion - Love Domes and Omnibolges