Generatepress is no doubt a great WordPress theme. This is one of the fastest WordPress theme available in the market. Like every other WordPress theme, Generatepress also have some bugs. One of the most annoying bug is the sub menu is not clickable and displayable while the official AMP plugin is installed and activated. I do not know whether it have problem with just GP free theme, but the problems arises with GP Premium + official AMP by google plugin.
There is a lot of support ticket available in the GP forum. There you can find only one solution that says to install a plugin form https://github.com/tomusborne/amp-for-generatepress. Obviously, this plugin works to toggle the amp menu. This plugin does not show the first item form the submenu as well as it is not clickable.
The solution is not provided yet by the developers of both GP Premium and AMP.
However, I got an Idea to overcome this problem. This problem can be eliminated by activating the Off-Canvas module in the customizer pane.
Step-1: Go to Customizer panel form WordPress Dashboard.
WordPress Admin > Appearance > Customizer
Step-2: Click on Layout then Off Canvas Panel.
Step-3: Click on Off Canvas option and select Mobile Only.
Now the problem is solved. selecting Off Canvas panel could solve the Generatepress + amp menu problem.