|MadSci Network: Astronomy|
The site you mentioned seems to me to be a parody of the sites that object to biological evolution. In particular the "only a theory" phrase is a reference to similar language inserted (on a disclaimer sticker) by school boards into biological textbooks in various places around the USA. This was done in order to appease certain pressure groups who have trouble understanding what evolution means and how it works. These groups also do not seem to understand what the word "theory" means and how it is used in science.
For a news article (one of many) about these warning labels, see http://www.ncseweb.org/resources/news/ 2005/AS/236_evolution_warning_label_remove_2_11_2005.asp
The theory of gravity is one of the most well established ideas in physics. To quote just one example, we can use the theory of gravity to calculate the positions of the planets to very high precision. Consider the act of sending robotic spacecraft (such as the Mars rovers or the Huygens probe on Titan) to land on other planets. If there were something wrong about our understanding of gravity, then we would not be able to get the spacecraft to the right places on time.
There are many meanings of the word "theory." In science, the word is used to denote a system of ideas that have been shown to be very useful in understanding some aspect of Nature. Other times, the word means the set of principles that are used to organize an extensive set of observations. In short a theory is the set of ideas we use to explain facts.
The theory of gravity, therefore, organizes and explains the facts of planetary motions, how things fall near the Earth, how tides work, and so on.
The theory of evolution organizes and explains the facts of changes in the types of living things through time.
In everyday speech, however, the word "theory" means something far less powerful. For example, if the pizza in the fridge is missing, you might hear someone ask "Who took it? What's your theory?"
Now those opposed to the proper teaching of science try to make people believe that the facts of evolution are not correct, and the theory of evolution is not a valid set of ideas. Much of the time, these people make false or incorrect statements about evolution, and then claim that the theory of evolution simply can't be true since it contains obvious contradictions or other problems.
All of these characteristics are brought out in this parody on the theory of gravity. The parody claims (incorrectly) that there are many problems in the theory of gravity, and therefore you don't need to pay attention to it.
It would take me a very long time to pick all apart the claims in the parody. But I can assure you that the theory of gravity does explain why there are two high tides each day, or how the Earth/Moon system orbits the Sun.
Try the links in the MadSci Library for more information on Astronomy.