On your Menu Particle's Settings tab add a custom class like "center-menu" or something then add this custom css
.center-menu {
display: flex;
justify-content: center;
}
Custom CSS via Admin:
docs.gantry.org/gantry5/configure/page-settings#assets
Custom CSS/SCSS file:
docs.gantry.org/gantry5/tutorials/adding-a-custom-style-sheet