If I am currently on my computer logged in via gmail and then login via your application, your app makes the connection between my gmail account and the account of your website.
If I logout, only my session on your website is destroyed and I am logged out from your website. This has no impact on my logged in status on gmail. If I visit the gmail website now, I will still be logged in via my gmail account. Therefore, if I again visit your website and login via gmail, I will be logged in with my associated account.
So if another person wants to login, I first have to go to gmail and logout. The other person has to login on gmail and can then login on your website with his gmail credentials.
How to solve this ? So that logout works for both gmail and my site.