I'm assuming that you just stare at her and you don't actually talk to her that much. If this assumption is correct, then the thing to do is go up and talk to her! I know that this will be kind of hard to do since the only class you have together is gym (and let's be honest here, there really isn't that much to talk about in gym), but you'll have to somehow try to find something that you guys might have in common and go from there. But if you can somehow find a way to talk about gym class (about the kids in gym, the teacher, or the subject) that is interesting and that she can relate to, then go for it. The point is to do some investigating to find something to talk about. And it's okay to be a bit awkward at first but the important thing is to have confidence while holding the conversation. Just imagine her being someone you're already friends with or someone you're familiar with and don't be shy to speak freely. Although be careful that when I say speak freely, I don't mean talking crap or about inappropriate things so remember that you still need to be respectful.
After getting some conversations started and you know each other a little bit, try to flirt a little bit. This should signal that your intentions aren't to be just friends with her but a little more than that. If she flirts back and things go well, eventually you can ask her to go out for coffee or to the movies. If that seems a bit too forward for you, you can ask her to hang out in a group of friends and later single her out and pay more attention to her.
For now, if you still don't have the courage to talk to her or don't know what to say, then try to refrain from staring at her too much because that may creep her (and frankly anyone) out in the long run. But if you do happen to stare at her and she catches you, just simply give a small smile and calmly look away. Don't just freak out and look away because to some girls that could be seen as creepy (to others, it may seem cute but it depends on the girl). If she responds well to this "smile and look away" technique, then there could be potential and you just need to know when to start talking to her. If she looks disgusted and creeped out then she't just not into you and you shouldn't pursue her anymore (especially if she rudely and immaturely shows disinterest such as giving you a disgusted look or saying something mean because there are clearly more polite and mature ways to show disinterest).
And about you thinking that she's "the one", there is a bit of a problem with that statement. I'm not saying that you're not right because you could be. I'm just warning you that that may not be correct so you're prepared for whatever the outcome may be. Most people nowadays do not find "the one" when they're in high school. You're still young in high school and you still have plenty of opportunities to meet new people. On top of that, you are still growing and changing and your preferences and personalities may change drastically once you're out of high school, so someone like this girl may no longer interest you in the next few years. This means that even if you do end up dating her, you may lose interest in her after a while. I know I can definitely vouch for the changing in preferences because now that I look back at the crushes I had during my high school years, I often go "what was I thinking?!" to most of them. Although true love at first sight can happen, it's rare. Infatuation at first sight however is quite common and infatuation doesn't always lead to love. Just be aware that you don't really know this girl and you may just be infatuated with her, so don't put your hopes up into thinking that she's "the one". I know that it's kind of fun to think about your crush as "the one" (heck I'm not in high school anymore and I still do that sometimes!) but just know that these are all things in your head and don't get too carried away.
Anyway, I wish you luck! If you have any more questions, feel free to ask :)