How to allow only selected user login in gae+python application?
You basically need to do it in two steps:

Do what systempuntoout's answer said to only allow logged-in users to see your site.
On each of your routes (URL handlers), the first step should be to get their user object and check if they are on a list you're keeping of users allowed to see your app. For a first run, you could just have the list be a global variable, but this isn't very flexible (it makes you redeploy your app every time you want to update the list), so for a second run you should refactor it to perhaps read from the Datastore to see if a user is in the allowed list or not.
I want to upload my application on google app engine
and want to use this by only selected user
so want to know how it possible.
want to use users gmail account.