A raw image is the unprocessed data from a camera sensor. More advanced cameras are able to store this pure data and not only the image in JPG format, which has the fancy look you can setup in your camera (natural, vivid, sepia, monochrome, ...). The raw images hold much more information about the photo than a JPG image and they allow photographers to recover a lot of details that would not be available in a JPG. A raw processor as it is mentioned in the announcement is a computer program that allows you to edit such raw photos. The fact that editing raw photos comes with so many more possibilities, the outcomes of the editing step can be very different. That's the motivation for this community activity: we invite all photographers to use the raw picture you can download (see the announcement text) and to edit it to their liking - and share it here of course. Seeing the different interpretations of the exact same photo is very interesting.
Have a look at the past months we have seen this activity going on so far. Just click on the tag #diaspoRAW and browse throught the posts.
I don't think Gimp can be used for editing raw photos directly, but there are other programs, free and not free, for the task. And of course you can always use any picture editing program at a later stage in the editing process - like Gimp, Photoshop, Affinity etc...