I'm not a pro with gargonians nor have I purposely fragged one. I did have a purple and more hard type that grows out here in FL and a couple of the 'branches' would sometimes snap off. These branches that snapped off were basically frags and continued to grow.
That being said...if you really need to frag it..you may be able to just cut off a part, or the whole 'branch' and that should work...although don't get mad at me if it doesn't because I'm not very experienced with gargonians other than only owning one ever
Cut off a branch, Scrape the flesh off of the bottom about 1/4 inch. Get an empty snail shell. Put the cut branch, scraped end, into the shell and some super glue gel. Then use the shell as an anchor in the sand, or glue it to a rock etc.