How to change Twenty Seventeen theme’s navigation bar location?

Update your .navigation-top class to use top: 0; so there is no gap for anonymous users.

.navigation-top {
    background-color: transparent;
    border: none;
    width: 100%;
    height: 120px;
    top: 0;
    font-size: 130%;
    bottom: auto;
    position: fixed;
    left: 0;
    right: 0;
    z-index: 7;
}

Then include an additional style for authenticated users which accounts for the admin bar.

.admin-bar .navigation-top{
  top: 32px;
}

WordPress core adds the admin-bar class the body tag when the admin bar is visible so you don’t have to add this class yourself.