Outlook 2007 keeps asking for username and password gmail

There are perform following steps:

We can Turn on "allow less secure apps" to access accounts
1. Sign in to your Google Admin console.
2. Click on Security Option

3. Click on Basic settings.
4. Under Less secure apps.
5. select Go to settings for less secure apps.
6. In the sub window, select the Allow users to manage their access to less secure apps radio button.

If you Can't find this option please click Here