I just saw this while browsing. There are plenty of tutorials around for this as it is a wordpress method that works with GP. It involves anchor links and custom menus…
In HTML this is called a document fragment.
There are two steps. First you insert an anchor in your webpage:
Then in Appearance > Menus, in the Custom Links box you put in the URL plus a pound sign plus the fragment name:
Then you can put the menu item anywhere in your menus.