You can do it simply:
import org.codehaus.groovy.grails.web.context.ServletContextHolder as SCH
...
def requestResourceForTwitter(Token token, String url) {
String appAttribute = GrailsApplicationAttributes.APPLICATION_CONTEXT
Object appContext = SCH.servletContext.getAttribute(appAttribute)
def oauthService = appContext.oauthService
Response response = oauthService.getTwitterResource(token, url)
if(response.code == 200){
return JSON.parse(response.body)
}
}