You want to change the following CSS style to:
width : 100%;
You can either define it in your own skin or your own stylesheet, or you can modify the following CSS file in the Default skin:
I think the Best Practice would be to define a new skin.
There is definitly a lot to digest with modifying and configuring GateIn.
Is there a tool for creating / editing skins? Or just take an existing skin as start tweaking the source?
You can use JBoss Developer Studio for editing the stylesheets of a skin, but I don't know of a tool that initially creates the necessary files for you. I've only done it once but I created a Dyanmic Web Project and imported the sample skin - that seemed to work fine.