Hi Anna,
Our plan is to have a yearly selection process for GEM (similar to our initial one) that decides which datasets we want to focus on. This year's process will start after the workshop in August. While I do not dictate the criteria, I can assume that we will prefer datasets with interesting and challenging test splits, good documentation and non-English language.
Besides that, even if not selected for the main challenges, it is fairly easy to make a task compatible with our evaluation framework and the one for creating challenge sets, which can be done independently (and we are happy to help).
Best
Sebastian