Gmail Checker restores functionality lost when Google discontinued the Windows Google Talk client in February of 2015. The Talk client would notify you when new emails arrived in your inbox, but there is no way to do this natively in Windows without running Chrome in the background. It is a simple application that runs in your system tray and slides a notification onto screen when a new message has arrived, it doesn’t blink or flash or do anything else.
Note: this app will no longer function with Gmail after May 31st, 2022. They have disabled the option to login with your credentials in 3rd party apps. There is no way to re-enable this functionality in your account anymore.
This is a known issue, but it is resolvable. You need to log into your Google account and modify your security settings to allow "less secure" applications. It's not recommended by Google, but it will allow the application to work. There may be better solutions available now though, so it may be best to let this app go by the wayside for security reasons.