how to check out a baseline of a component via eclipse GUI?

In the Eclipse GUI, open your workspace. In the Editor of the workspace you should see a Components field, where all the components that are defined to be loaded into your workspace are visible. With the buttons right of this field, you can add components or change them, i.e. change the baseline to be loaded to your workspace. Make the appropiate changes and save theses to your workspace.
When you load the workspace in your Eclipse GUI you should have all the content marked with the selected baseline in your local workspace.

Hope that helps,

Henning

You can get the baseline of a component by configuring the pending changes view.