I think if an rp is still within the interest check phase, then without any GM permission there shouldn't be any effort by players to take it over. Use the interest check for inspiration, but go make your own instead and actually [i]make it your own[/i]. 

But for rps that are already underway, and ones in that have been going on a long time in particular... if a GM just vanishes for whatever reason, then... I suppose after a grace waiting period, I think it would be okay for the players to continue on, whether by deciding on a GM to lead up until the moment the actual GM returns, or to just keep soldiering on as best they can. If the players have put in the work and effort and enjoyment to get that far into an rp, why should they be forced to stop because the GM dissapears?