Ah – you found a bug!
This has been fixed in the next version of GP.
If you’d like to apply the fix now, add this to your CSS:
@media screen and (max-width: 768px) {
.nav-aligned-right.nav-below-header .main-navigation .menu > li,
.nav-aligned-right.nav-below-header .main-navigation .sf-menu > li,
.nav-aligned-right.nav-above-header .main-navigation .menu > li,
.nav-aligned-right.nav-above-header .main-navigation .sf-menu > li,
.nav-aligned-center.nav-below-header .main-navigation .menu > li,
.nav-aligned-center.nav-below-header .main-navigation .sf-menu > li,
.nav-aligned-center.nav-above-header .main-navigation .menu > li,
.nav-aligned-center.nav-above-header .main-navigation .sf-menu > li {
display: block;
margin: 0;
text-align: left;
}
}