Nice! At a total of about 35-40 refreshes this orbfest overall and I still haven't had that happen, odds have to be incredibly low. No idea if it's accurate, but the numbers floating around on reddit for chance of a Gigantaur was like ~3%? That's super lucky!
My luck this orbfest so far hasn't been as good, I'm at about 10 refreshes (away from my PC/notes) and I didn't get a single 5* drop until about 6 or 7 in, but I've gotten 3 or 4 in the last 5 refreshes or so. Still hoping I didn't use up all of my luck last phase >_>