Are you using GP 3.0+ with the Flex option?
If so you can try this CSS that is used in Sider:
@media (min-width: 1000px) {
body {
padding: 0 50px 50px;
margin-left: 300px;
}
.site-header {
position: fixed;
left: 0;
top: 0;
width: 300px;
z-index: 300;
height: 100%;
overflow: auto;
overflow-x: hidden;
-webkit-backface-visibility: hidden;
-webkit-overflow-scrolling: touch;
transition: .1s ease;
}
.admin-bar .site-header {
top: 32px;
}
.site-header .main-navigation li {
float: none;
}
}
.inside-header {
flex: 1;
flex-direction: column;
}
.header-widget {
order: 3;
text-align: center;
}
.nav-float-right .inside-header .main-navigation {
order: 2;
margin-top: 50px;
margin-bottom: 50px;
}
.dropdown-click .site-header .main-navigation ul ul {
position: relative;
}
.main-navigation .main-nav>ul {
flex-direction: column;
}
.nav-float-right .header-widget {
margin-left: 0;
}
.nav-float-right .header-widget .widget {
margin-bottom: 50px;
}
.nav-float-right #site-navigation {
margin-right: auto;
}