The Gator stack can be ‘fixed’ to either the top or bottom of a webpage too. The option is provided within the Positioning menu, in Gator Navigation Bar Settings.
However I admit to not being a huge fan of fixed navigation bars. They can break the normal tab / content flow, consume extra screen ‘real estate’ and make scrolling more jerky in older or under-powered web browsers. So you might find many other themes like Boutique, RWSkinz, Volcano and Wilderness provide the option of a fixed navigation bar, but it may not be enabled in the theme settings by default.
Slightly off-topic; I very much like this method of making navigation bars ‘sticky’ with pure CSS: