Tough loss. I've been there. Twice now. It stings nonetheless.
Mags and gigs are profoundly different anemones when it comes to treatment. Mags typically look terrible, then perk up, and go on an uphill swing. Gigs go up and down. However, when they're down where they fully deflate and look terrible -- flat, no tentacle movement, mouth open -- they never come back -- at least I haven't seen one recover, ever.
I currently have two gigs in treatment (in separate tanks), and it really is day by day with both. I also noticed that Cipro alone doesn't work with these two nems. I wonder if the gigs that arrived recently were all sick with the same bacterial infection that doesn't respond well to Cipro. I have to use "shock therapy" where I hit them with Septra and they fully deflate, yet come back better the next few days, so I treat them with Cipro. Then it happens again. One of the gigs appears to stress spawn during treatment as well so I have to do a lot of water changes. I don't know if this negatively affects them as well. I try to match the water as best as I can, but I wonder if the change in parameters stresses them out even more.
Anyway, my point is that Cipro is not the magic potion that cures all anemones. It works -- most of the time -- but I think this recent crop of gigs were hit hard so there wasn't much you could do to save your nem. If it's any consolation, I went back and looked at the photos of when you thought it appeared healthy, and even in those photos you can tell that it's not healthy -- the tentacles are short, though somewhat extended, the color appears pastel, and the overall body appeared compacted as if it wasn't fully inflating.
Trust me -- you'll have another chance at a gig. You may just have to be patient. I would prep your QT rack as you mentioned, but go with 10 gallon tanks -- I think the size of most gigs requires at least a 10 gallon tank. When you find another gig, you'll have Cipro and Septra on hand. After my first gig died (in the pre-Cipro days), I found a second gig a few months later, and it was with me for over two years before I found a new owner with a larger tank to house it. I think your second gig will thrive just like mine did. Good luck!