Let's assume life evolved on an impossible planet where gravity was essentially reversed. Everything solid falls towards the sky and into outer space, while liquids and gasses fall down, towards the ground. What interesting and unexpected creatures could have evolved here? Can you describe some of them and tell me how they have successfully adapted to exploit this unusual environment?
To start, it is not technically feasible for gravity to be “reversed.” With the way gravity works, there is something known as the Inverse-Square Law. Isaac Newton suggested that the force of gravity acting between any two objects can be calculated to be inversely proportional to the square of the separation distance between the object's centers. Essentially, if gravity were to be reversed, the gravitational force would increase the further away you moved from the planet’s core, and the planet would end up exerting so much pressure on itself that it would implode. Factoring this into the entire universe, and you would very quickly find that you wouldn’t have a universe at all. 

That being said, we can entertain the idea for purely speculative purposes. It is difficult to imagine what life might look like if it had evolved on an alien planet where gravity was reversed, but here are a number of different adaptations that may have taken place in order to allow for such a phenomenon to occur.

Adhesive Contact: One possible adaptation life might take on such a planet is evolving adhesive pads or suction cup feet. This would allow the animals to stay grounded despite gravity pulling them up.

Subterranean Creatures: Something you might expect to find on a planet with this predicament is a higher amount of animals living underground. It is possible that entire ecosystems would have evolved below the surface to circumvent the problem of drifting off into space. This way, the many creatures that exist underground on the planet could simply live their lives upside down as if they were under regular gravity.

Air Buoyancy: Another odd thing that you might find on a planet in which the gravity is reversed is creatures filled with helium. The same way a balloon rises because it’s lighter than air, you might expect it to fall if the rules of gravity were reversed. This is because, as you stated, all of the rules of gravity regarding mass would be flipped, so instead of gasses being a light state of matter, they would instead be heavy.

These are just some possibilities of odd adaptations you might expect to find on an alien planet with its gravity flipped. Of course, in our universe all of this is impossible, but it’s nonetheless entertaining to imagine how it might pan out if gravity worked differently.
Why would the planet implode if the gravity is going to be pulling it apart?