If you just installed template on existing joomla i think what you need to do is go to template settings then Fonts .
And do below
Headers - font : Google Fonts
Font url: http://fonts.googleapis.com/css?family=OswaldFont family: Oswald
Headers - selectors: #gkMainMenu
in the row where it says (Headers - selectors) if you have some other stuff there just add #gkMainMenu ..
I installed the quickstart and mine looks like this
Headers - selectors : h1,h2,h3,h4,h5,h6,
.result-title,
#gkPageTopLinks,
#gkMainMenu,
button,
.button,
input[type="submit"],
input[type="button"],
.page-prev a,
.page-next a,
ul.gkTabs li,
.gkIsMoreNews,
.vertical .nspInfo,
.nspHeader,
.pagination,
#gkFooter,
.nspMain .readon,
p.numblocks span,
table th