No such thing as a stupid question. It's just a very tiring question to answer. It's been answered on various other forums, but not here. This forum is a bit young.
The reason no one answered, is because of the paradigm terrain modelling industry works in. All terrain modelling algorithms in circulation right now, work with "heightfield" data. Which means all terrain tools also work within this constraint. And THIS being a forum dedicated to a tool called Gaea, assumes that the question would be about Gaea, a terrain modelling tool that works with "height fields".
What does it mean for a beginner you ask? It means that: "All terrains created with Gaea, will be a flat plane, with polygons displaced only in one axis (up)".
Why am I explaining it? How does it relate to the original question?
THIS is the floating islands scene you're talking about. It's a complete 3-dimensional surface detail model. The rock holds data in all directions (not just one, like I described above).
THIS is what a Gaea terrain looks like, in the hands of a very skilled terrain artist. In this case, someone from the Quadspinner team. Notice how the terrain holds detail only on one side, UP.
So, going back to your original question @-Karg-
-Karg- Hello, I would like to know if it is possible to make a floating island, a little in the style of Avatar?
Answer: It IS possible, but not in Gaea, or any other terrain modelling exclusive tool.
What you need, is a full 3D content creation suite, or a 3D sculpting tool, to accomplish such a thing. Blender is a free program you can try.
Hope this explanation helps, somewhat!