hello nroon, it's probably better to contact the support of the site where this warning is shown, since only they will know what's going on & can give you detailed advice...

Can you give us a link to the website nroon?

We detected activity on your Google Account from a location you don't usually sign in from. Review the information below and tell us whether you recognize this activity. Learn more Event Description

Details

Time Application or device sign-in attempt Prevented

Argentina speedy.com.ar, IP Address: 190.175.93.254

Nov 24, 2012 9:42:11 PM Application or device sign-in attempt Prevented

Mexico prod-infinitum.com.mx, IP Address: 187.233.132.32

Dec 2, 2012 1:02:50 AM Application or device sign-in attempt Prevented

United States swbell.net, IP Address: 69.154.77.197

Dec 10, 2012 9:49:57 AM

I see what you mean. That does look bad. I was thinking this could be fake but it does seem real at the same time. I am guessing you only see this message when logging in with Firefox?

Can you try to open Internet Explorer and login to your Gmail account with that? If the same message appears in Internet Explorer, I believe the threat may be real. In that case, I would change my password. But not by clicking on any links in those messages. Instead, go straight to your account settings and change the password there.

To change your Google account password:
https://www.google.com/settings/security?hl=en

Notice the HTTPS at the beginning of the link, that means it's very secure.

Anyway to be safe, use the link I gave you to change your password. Also use Internet Explorer or Google Chrome to do it, if you believe Firefox was infected with a virus.

Also read this page about fake Gmail alerts:
http://techie-buzz.com/online-security/gmail-suspicious-sign-in-prevented.html

Does your warning look like this?