Version 2.15 is in "dustin" overlay.http://gpo.zugaina.org/app-admin/keepass-bin
This means you have to add the "dustin" overlay to Portage, and emerge it.
Are you familliar with Portage and overlays?
You could also download the .bin file on the same page, and execute., but that's NOT supported by Sabayon.
If you wish, you can make a package request.https://bugs.sabayon.org/
You can use the same loginname and passwd as you use for this forum.