Add this code in custom.scss file or Gantry Page Settings:
#g-navigation .g-main-nav .g-toplevel > li:not(.g-menu-item-type-particle):not(.g-menu-item-type-module):hover {
background: transparent;
}
To add custom style sheet to Gantry 5 Framework please follow these instructions:
docs.gantry.org/gantry5/tutorials/adding-a-custom-style-sheet
or if you want to add them in the Page Settings:
docs.gantry.org/gantry5/configure/page-settings#custom-css-js