It looks like the plugins in question in this post were hosted on Bintray (which has been shut down).
Since the post is a couple of years old, it’s probably due for a refresh. I’ll add it to our todo list!
In the meantime, if your Grails application uses Spring, you can take a look at the Spring Security integration (assuming it supports Spring Sec 5.x+).
Otherwise, you can take a look at this technique (which works great for new apps too):