While I don't think this is a great storyline by any stretch, I think Dini pulled off the whole "bring back an obscure-as-hell forgotten Silver Age character/concept" better than Grant Morrison has, by and large.
When Morrison does it, it's almost as if he expects to you already be familiar with stuff like Zurr-En-Arrh, or to make you do research. When you introduce a new element into your work, you can't just expect the reader to know what it's supposed to mean.
But Dini pulls it off better in two steps. First (in the page that wasn't scanned here), the character is revealed in such a way that says to the reader, "Hey, this is important! Bum-bum-BUM!" And then immediately after, we have Selina saying, "Never heard of you," thus becoming the voice of the reader who is likely totally unfamiliar with the character, which opens the door to a quick explanation of backstory, and bada-bing bada-boom, everybody's up to speed in the course of one single panel (ideally; the risk here is getting too much exposition).
Whereas with Morrison, you need to either be a big flaming nerd who knows obscure stuff so the stuff can have any meaning, or you need to pick up a copy of BATMAN: THE BLACK CASEBOOK which collects all those obscure stories to catch you up to speed. Which I just find smug and elitist on Morrison's part, not to mention poor storytelling. Unless, of course, you like the puzzle of finding all those obscure stories, in which case god bless ya.
Again, not saying the above is brilliant or anything. Just saying if you're gonna do it, that's how it's done.
And for the record, the only way I was familiar with Gaggy at all was from this cover, which was reprinted in the original GREATEST JOKER STORIES EVER TOLD trade paperback: