Google rolled out a new security feature
recently that you have to be aware of.
Some background - how Internet works
When a computer connects to the Internet, it is been given an IP address (Internet Protocol address) that looks like 192.168.9.10. This IP allows clients (PC, phones...) and servers (Internet servers) to communicate. The client request the IP server a web page giving him its IP address and the URL it wants to get and the server send it back the web page at this URL.
Then the content is transported in wires through "routers". Routers have several input-output plugs in which wires are connected and know to which cable they have to send a particular data packet according to its IP address.
To be manageable, groups of IP addresses are reserved for a country, a provider, an organization, a company.
These are all "public IP addresses". They have to be uniquely given for the Internet to work.
The web is a large network of interconnected wires (and sometimes wireless communications). If one can associate a group of IP addresses to a given router and if he knows where is geo-located the router, then he knows more or less where is the hardware using this IP address.
Large databases of IP geolocation
have been built.
Google security improvement
Based on IP geolocation, Google may detect that your account is being accessed by someone who is far from where you generally access your Google account. This can be seen as a suspicious activity on your account, like it is done with the use of credit cards for instance.
In this case, Gmail warns you. If you think your account is being compromised, then you have to log-off all other computers using your account (there is a menu at the bottom of the gmail page for that) and renew your password (use MyIdentity service)
For detailed information on this feature, check the google online security blog