The following is from the version 2.5 help file.
The Free Gravity FAQ
What does it mean that Gravity is free?
It means just that, that you don't have to register or pay for Gravity to use it. You can use it however you please, you can re-distribute it, you just cannot sell it.
Why is MicroPlanet making it available for free?
The basic answer to this is so that we can feed our families. There are numerous market factors (free alternatives - including those bundled with Windows itself, widespread piracy, etc) that make it impossible to make money on a USENET newsreader. Even though Gravity still generated some income, it was not enough to develop and support it. It was an agonizing decision to make. We are trying to be as fair as possible to our users. For example, we have never had a revenue upgrade at MicroPlanet (Anawave did for version 2.0 when they owned the product) - all features we added were free to our customers.
What are the policies regarding current customers?
No one who bought Gravity on or after November 1, 2000 has been or will be charged for Gravity. We were in the process of deciding what to do and did not process any of the transactions. Users were added to the registration database, however.
Is email support available?
We can no longer provide email support (corporate licensees, see the policies section of this FAQ). In fact, because of outside obligations, we were unable to provide an adequate level of support in Q4 of 2000. We sincerely apologize for that.
Why did MicroPlanet decide not to use an Ad-generated income model for Gravity ?
First and foremost, we do not like ad-ware. Second, if we continued to accept money for Gravity, we would be obligated to support it. We do not believe that there would be sufficient income to provide support and ongoing product improvements.
I lost my registration key. Can you send me another?
You don't need a registration key for Gravity. Download version 2.5 or after - it is free for everyone.