The pilot investigation and the actual investigation are the steps of GEMS that students are likely to identify as the most enjoyable part of working scientifically.
A trial run of the science investigation flushes out unexpected and unforeseen issues around teamwork, the method, equipment, recording data etc.
The role of mentors
Between the first and second school visits, mentors may be available to advise students online if you arrange for this to occur.
You could set up a class wiki with each group having a separate page, invite mentors to join the wiki and monitor student-mentor interactions. The document How to set up and use a wikispace which was created by a MyScience teacher provides a detailed guide to setting up a wiki. You may need to provide a lesson on 'netiquette' for students before they interact online with mentors. Students could report what happened in their pilot investigation to their mentor and ask for advice. They could take video clips or images and upload these, or even communicate in real time if this can be arranged to suit participants' schedules.
Mentors are not expected to be constantly online responding to multiple questions from every group member. One group member could be responsible for gathering the group's questions and sending them (email/upload to a wiki) to their mentor. The mentor could then respond with answers to the questions within an agreed time frame. Alternatively, you may prefer to have all questions and answers channelled through you.