Its possible you may want to show a single gallery in the menu, you may only be showing one for your portfolio, or you may want multiples as a drop down menu for ease of access for your users.

Once you’ve created your galleries, you can go to Appearance >> Menu’s, and from here you will want to go to the top right, screen options, drop down the menu and select galleries. (This area will show you all available options for the menu).

Now that you have the Galleries option selected, scroll down, and in the menu area you will see all the options, drop down galleries:

You can now select a gallery, or multiple galleries to add to the menu.

If you want to add some options in a drop down menu, you can drag them one iteration to the right:

Then you can save the menu, bottom right.