launch a .stl file in gazezbo

edit

You should create a model for that...

You should have a llok at the buiild a robot tutorial series:

http://gazebosim.org/tutorials?cat=bu...

If you just want to visualize it this might be the tutorial that answers your question:

http://gazebosim.org/tutorials?tut=im...

It contains a simple World file that you can use...