If you'd like to reach GP-100 owners (a generally friendly and helpful bunch) to ask questions or exchange ideas, use the gp-100 users mailing list. Sending mail to firstname.lastname@example.org will send your message to a group of gp-100 owners. If you'd like to join the group, simply send a message to join the Roland GP100 Users mail alias! with "subscribe roland-gp100-users" in the message body.
If you're sending mail to the mailing list, but aren't a member yourself, be sure to mention that in your message, so that people who reply will know to send the response to you directly, rather than to the list. Otherwise, you won't see the replies.
Here is a link to archives of the mailing list.