I have been running gnome-3.24 by unmasking the big gnome mask in portage for 3 weeks without any issues. Unless there is some issue I am not aware of it is probably a good idea to unmask it so ~ users can test it.
1) There is no gnome-3.24 in any gnome-overlay referenced in the summary 2) Many many 3.24 versions are still missing and needing work, see status report in already existing gnome-3.24 bug I'm marking this a duplicate of. *** This bug has been marked as a duplicate of bug 611270 ***