If you are using WPBakery Page Builder in your theme, you can also enable WPBakery Page Builder for Groovy menu blocks.

By default, WPBakery Page Builder (formerly, Visual Composer) is only enabled on page types. To enable the builder for Groovy Menu Blocks navigate to Admin panel > WPBakery Page Builder > Role manager tab > Post types select. Then switch Post types select to Custom and check necessary checkboxes below, gm_menu_block in our case. After that hit Save changes button at the bottom left of the page.