While I wasn't in a similar situation myself directly, I had an (online) friend that found himself in a similar situation. I tended to present much more aggressively ("male", some would say), but I was also quite outspoken so if somebody I expected to stick around assumed wrong (or even worse, expressed utter surprise that a "girl is playing videogames"), I'd have no problem correcting them (occasionally accompanied by a mild profanity to hammer in the point) right off the bat.
However, I had a very gentle and introverted friend that, especially next to me, presented very much like a female. I'm not sure if he was quite sure where he stands on his gender and sexuality at the time, and I think a somewhat gender-less world of online gaming suited him because of that. His natural way of expressing himself was also very gender-neutral, he would join the voice chats required to coordinate gameplay, but never actually talk (only the leader really needs to talk, the rest just need to listen and written acknowledgements are fine).
So he found himself in a situation very similar to yours -- he simply didn't correct a lot of his casual acquaintances. Which was fine until he became a close friend with another player and felt this got in the way of their friendship. He eventually went for the direct approach suggested in the other answers. They simply had a conversation.
I think what made it easier was realising that if his friend didn't want to continue this platonic relationship after he told him he was male or couldn't sympathise with why he didn't correct him straight away, then this is probably a friendship he doesn't want to continue himself either. So it might help you to think about what you are really worried would happen: damaging your relationship because you lied, the relationship changing once they find out you are female, or just drawing attention to yourself, all of the above or something completely different.
Still, in his situation it was only one person that he really cared about correcting, and for you it's a whole group. It takes a lot of courage to announce something big like that in front of a group -- and it's normal that not everybody can muster the same courage in all situations. But maybe dividing the task into telling individual people might be less intimidating. It also seems you are talking about a forum-type site, so the communication is mostly asynchronous. You can write a message while you feel good and comfortable, and check for the answer only when you feel prepared. I would suggest starting slowly, with a member that you feel the closest with. Considering the nature of online communication, I'd go for something short like:
Hi, I actually wanted to tell you something pretty big. A couple of years ago when I started posting here, you all assumed I was a guy, and since I was insecure about how you would react, I didn't correct anybody. Now, two years later, I am really enjoying my time with all of you and I know nobody would've cared if I told you from the start, but it feels a bit intimidating to announce to everybody at once that I'm female. I'm telling you first so I could have some support if needed, but I'm figuring out a way to share this with everybody in the next few weeks. Hope you're okay with this?
Then wait for their response. If the community really is as open as you say, this will get you a small safe place of at least one person who knows, and then you can go from there. Find the next-closest person or a few of them, and send a message similar to above, replacing:
I'm telling you first so I could have some support if needed, but I'm figuring out a way to share this with everybody in the next few weeks.
with something like
I've only told SoccerFan64 for now, but am planning on telling the wider community in a few weeks. I hope you wouldn't mind keeping this between yourselves until I figure out how to do that?
After doing this a few times, it should become less stressful and you'll have explained the situation to the members you're closest with. At this point, you could continue contacting people personally, do something more community wide if you feel more comfortable, or just tell the people you already talked with that they don't have to feel like it's a secret any more, as you don't really want to draw attention to yourself by posting a new topic just to announce this, but will start correcting people from now on and let the information make the rounds naturally, or something along those lines. Or just joining the announced voice call and saying "surprise" while knowing there'll be a number of people there to confirm to everybody that you're not joking. Whatever you decide, I think it will be much easier to tell the "community" after you've told some individuals first.