Generally how we handle things here is that the owner of a roleplay maintains ownership of that roleplay, we don't give it away even if the owner leaves. That said there's three things you can do.
The characters and contributions you guys made to the roleplay are of course yours, so you could make a new (but different) roleplay with a similar premise, and import your characters across and adapt them and their histories to work for the new roleplay. This involves the most work, since you can't take the original GMs stuff, IE their characters and IC organizations, and specific elements.
However the GM brought you guys into the roleplay, and as far as I know left no explicit instructions on what you can't do regarding progressing the story. That means you guys are free to just keep trucking on without the GM actively present. As staff though we won't grant outright ownership of the roleplay, but neither are we going to tell people 'no you can't keep roleplaying in this roleplay'. Just means if the owner returns and there's a conflict over the roleplay, the staff will generally back the original creator that it's still his/her roleplay.
And lastly you can always shoot a PM to the owner and ASK if they can turn the roleplay over to you, and invite a moderator or administrator to the conversation. Then if they return later they can't pull it out from under you. It was officially handed off. If they have their inbox rigged to e-mail them when they get PMed they might respond. Either way I would make this your first step because it's the least likely to cause you complications down the road, and then decide which of the other two options you guys want to pursue if you can't get a hold of the original GM.