By default, bootstrap CSS and JS is exist in the folder grails-app/asset/ stylesheet and grails-app/asset/javascripts respectively. Or you download latest bootstrap from http://getbootstrap.com/getting-started/ and after unzip, all CSS put grails-app/asset/stylesheet and jquery and bootstrap.js files will put in grails-app/asset/javascripts folder.
At now add CSS file in the layout/main.gsp file like as
<asset:stylesheet src="bootstrap.css"/> <asset:javascript src="jquery-2.2.0.min.js"/> <asset:javascript src="bootstrap.js"/>