We used Gamecenter because me and my partner didn't really have the expertise of setting up the server side that would be required for an app like this, so we decided to let Apple take care of that for us. We also didn't want people to have to make new usernames and passwords and have to find their friends again in a new service.

Thanks for those, I will keep all of this advice when we are ready to submit the next update.

Ah, I did not notice those. They are most likely the default Gamecenter multiplayer mode where all the participants are connected to the "game" at the same time. We found a lot of limitations in this because you could only have one "game" connected at a time and this was not feasible for a reminders app. The new turn based game mode worked best for us because not everyone had to be connected at once for you to start and you could have multiple "game" sessions going on at once.

By "organic" they most likely mean carbon based, which leads me to believe that it is most likely petroleum based.