Users from Germany and the UK sometimes have interchangeable "googlemail" and "gmail" email addresses.
Please be aware that if you have registered your Lookout account using a "googlemail.com" email address you must use that exact address to log into your account subsequently. You cannot use a corresponding "gmail.com" address, even though they may both be connected to the same Google account.
You can, however, change the email address for your account once you are logged in. Instructions can be found here.