Are you looking for supplementation? Do you need courses to be accredited somehow? If so, with whom? Are you looking for an online school that would replace your child's current school situation? Are you looking for something free?
I have four gifted kids, three now graduated from high school in a remote rural area with no gifted program, and we have not been at all impressed with online courses. That's not because of lack of challenging academics. There are plenty of courses out there, including lots of free ones, that would be at an appropriate level (Athena, Coursera, Udemy, KhanAcademy, Open Learning college courses etc.). We've also tried three different online publicly-funded high school programs here in BC, Canada.
But the thing is that for the most part, online learning utilizes the worst of the schoolish mode of learning: it's sit-down stuff, other-directed, done in a solitary fashion with minimal if any personal relationship underlying it, and is punctuated by a lot of busywork. Yes, some courses have virtual classroom time, some have online discussions and so on. But these things were just not sufficiently like group learning and personal relationships to take the place of their real-life counterparts. Even with my kids being quite introverted and very experienced with social media and online communication, online schooling felt very remote and isolating.
My kids have found it worthwhile and tolerable to do occasional online courses at the high school or college level to supplement what they're doing in the rest of their high school educational lives. Athena and Udemy have offered a couple of the better ones they tried. But these were just supplements, mostly used to facilitate access to other opportunities by proving mastery or to tick off some hoop-jumping requirement quickly and efficiently. I can't imagine them doing online courses as their entire educational program day after day. My incredibly disciplined eldest had to do more than half of her 12th grade course-load via online schooling due to situational issues; she managed, but if she'd had it to do over again she definitely would have tried to find a different way. Thankfully she was very busy with creative pursuits and all sorts of real-life adventures and that kept her sane.
I don't want to be completely discouraging, but I think that if you are considering full-time online schooling you should think realistically about what it's going to feel like to spend 4 to 6 hours every day in front of a computer screen with no peer support and little meaningful interaction with anyone.
Then again, maybe that's not what you're asking about. Some clarification would be helpful.
Some good suggestions above. I'll add our (somewhat limited) experience. My oldest is entering 5th grade so I can't comment on middle or high school, but we've done some online learning to supplement public schooling. Our district's TAG programming starts in the 4th grade, which is a long time to wait, and effective in-school accomodations were scattered and incomplete previously. 4th was better because she was in a split-level TAG class so all students were identified gifted (which is generously applied to the top 5-7% here) and was in the lower grade level of the split.
What we have done that's worked:
For mathematics, we use Dreambox, which I recommend because it's both gamified (so looks more like "fun!" to them) and pretty decent at teaching concepts. It won't teach any standard algorithms so won't replace an actual math curriculum, but algorithms are easy to pick up if you've got the conceptual background in place. Also, it's reasonably flexible and responsive to the child's performance and has done pretty well at leveling them up in any skill to where they "should" be.
If you want a math curriculum to actually replace school, for pre-algebra and above, try the Art of Problem Solving. Your student must be able to type pretty well because the live class session is entirely text-based. It really will take 5-7 hours a week for gifted mathematicians to do a class. I spoke to DD's TAG teacher about having her do her AoPS math in place of group math next year and believe that's what we'll be able to do because DD is the only student in TAG in her school ready for pre-algebra.
DD did a literature class through the Johns Hopkins CTY last summer that was well-done, if pricey. I've heard these classes may be a bit inconsistent due to varying instructors, but the one we had was good. But, it WAS pricey and I felt like she will get the same thing in her TAG class now that she's in it. At the moment she is instead working with a writing mentor every other week via Skype.
I've heard good things about Athena's and Online G3 if you're homeschooling full-time. But we're in public school and the times haven't ever worked out right.