If this is a site that is online could you please supply a link so we can better diagnose the issue?
It makes our suggestions more accurate if we see the actual coding of your site, as my test templates don't have a gap.
i tried different movements but the gap stays there even tough the menu disappears because it goes under the navigation i'll put -25 for you to see what i mean, I even thought it had something to do do with the height of the menu bar but i can't make it larger so I really don't know what i can do.
I have similar problem of gap between navigation menu.
I am using nebulae theme on wordpress network and face this problem on some subdomains and on some
subdomains its works fine.
I tried all of above solutions but none of them helped me to solve the issue.