Greenwich is a borough in East London, there is an old museum with the line on the ground that defined the meridian. You can visit if you come across London.

GMT, namely Greenwich Meridian Time is not actually the time of the meridian of Greenwich for half of the year. When you find yourself working with GMT, you really need to think about how you got there because it's wrong. 90% of the time you meant to use UTC and the other 10% you meant Europe/London timezone.