One of the issues some people have while installing GAP is python version:
$ Warning: You are using a Python runtime (2.6) that is more recent than the production runtime environment (2.5). Your application may use features that are not available in the production environment and may not work correctly when deployed to production.
Fortunately (in your face noobunters) Debian still have 2.5 on their reps, so to run the gap server correctly just install:
> apt-get install python2.5
Once it's done some people like to go and do weird python version switch renaming/updating-alternatives... and some other stuff that i don't like.
If you want to keep things simple and run 2.6 on your system and just want 2.5 for gap, just leave everything as it is and start your GAP server this way:
> python2.5 google_appengine/dev_appserver.py path/to/project/
ps: am i the only one who thinks the new /_admin/ panel added in the recent versions is the greatest thing ever?