Google GMail has a facility enabled by default to lock access to your GMail account if it detects you signing in from a different location or device.
Often internet users with a dynamic IP address, which is the norm with most broadband providers will experience this issue if the internet provider assigns them a different IP address. Also if you have a GMail account on your mobile phone, as you move around you connect to different cell sites and Google will incorrectly detect this as a security compromise.
It is possible to disable this warning if it is troublesome but for your own security, it is advised not to.
Click the Details link next to the Last account activity line at the bottom of Gmail page. Change your Alert Preference there.