Easily the best 2D puzzle game I've ever played.
Given the game-mechanics, it's impossible not to compare this game to (the puzzles in) Portal 2. The lack of a third-dimension makes the portal-only puzzles much easier than in Portal 2. However, Gateways also adds several game-mechanics not found in Portal 2, such as the ability to change gravity or travel backwards in time. The new mechanics are introduced continuously throughout the game, keeping the puzzles fresh and interesting. Additionally, they ensure the puzzles are challenging even to Portal-veterans.
My only complaint about the game is that it's so short (~4 hours of gameplay). However, when that's my only complaint about your puzzle-game, you're in very good shape, especially for only $5.