I am HSing, and I don't know anyone who is HSing full-time and using K12. (That includes both GT and ND kids.)
That's not to say it can't be done or that you shouldn't do it. It's just that it's not something you *need* to do, even/especially with an HG+ child.
What I see as the joy of homeschooling, especially a gifted kid, is the ability to follow the child's lead and explore things of interest rather than to get locked into someone else's curriculum. You have to log into the computer and keep track of how many hours you teach on each subject and keep track of which lesson she's on and the teachers really do check on you. There is a lot of flexibility, but you are still working with their plan, not yours or your child's.
I know it sounds scary to just go out on your own, but there are plenty here and elsewhere who could help you.
I agree completely with acs! Don't get locked into thinking that you must do school at home in order to teach your child. You don't. I promise you, HSing is a lot easier than you think it's going to be. The curriculum has been by far the easiest part of HSing for me, to tell you the truth. You just follow your child's interests, visit the library a lot, and get in with a good HSing group. That's all easy (and not expensive!). The hardest part for me has been getting enough time to myself, but then I'm a deeply introverted person and I have a 6.5yo and a 3.5yo, so your situation may be significantly different than mine there. But planning the curriculum? No big deal!
If you have specific fears, questions, etc., I'll be happy to share what I've learned. I'm sure Lorel would help, too--she helped me when I was where you are. But there's lots of stuff that works and that doesn't require that you to spend X amount of time on any subject.
Singapore Math, Aleks, and Saxon Math are all reasonably priced math programs that tend to be popular with HSers. EPGY is more expensive, but is well-liked, too. Though I'm not using any other packaged curriculum but math, there are also programs that come highly recommended by friends (both virtual and IRL) for other subjects.
If I may, I have two suggestions for you--the best pieces of advice I got before I began HSing:
1) Start with why you're HSing, what you want your child to learn in the broadest sense, how your child learns, how your personalities will fit, etc.--the basics! Don't start with decisions about specific curricula. You're putting the cart before the horse, and it's hard to judge a curriculum if you don't know what you want your child to know by the end of the year and why you value those things. Also, make sure you know the law for your state. What's required of you in terms of testing, reporting, etc. may affect your choices. Spend 5 minutes learning the basics of the various schools of thought of HSing: unschooling, eclectic (that's me!), Charlotte Mason, classical, school at home, etc. You'll figure out pretty fast what suits your family's style and needs and what seems totally wrong for you.
2) When it comes to purchasing/signing on to a certain curriculum, *LESS IS MORE*! You can easily commit to too much packaged curriculum and then find that it doesn't work for you and you're stuck with it. OTOH, it's pretty much impossible to have too little packaged curriculum at first, because if you find that you're lacking, you can buy/commit then.
Keep in mind that all your child really has to do is learn a year's worth of material for a year's worth of work. For these HG+ kids, that would probably happen if we locked them in a room alone with a pile of books for a year! I know you're used to having to fight for every scrap of learning your DD gets, but HSing is a totally different animal. My DS6 and I spend maybe 3 hours a day, total, on school (and that includes dawdling time and tidying up time). The rest of the day, he plays. He's finished two years of math in less than 5 months, and I've been trying to go deeper, not faster with him, so I've been slowing him down. He's reading about 3 grades higher than he was at the end of last year, and we have no packaged reading curriculum. He's using the scientific method and conducting experiments, and his public school classroom is just now reading a thermometer!
I'm telling you, it's soooooooo much easier than you think it is!
Stepping off soapbox now...
Please let me know if I can help you in any way.