Hmm, I don't really think you'd need to use external pages at all. How about changing your intro page into a static page with no blocks? If you have GL 1.3.8, Staticpages 1.3 has
this built in.
If you haven't upgraded yet and want to go about it the old way anyways, here are some earlier threads about splash pages:
here,
here, and
here
For your login/logout link, use this
code:
(The code tag added
Text Formatted Code
<br />
after every line when I tried posting this as code, so I just linked it instead. Must be a forum bug?)