He somehow slipped it out without me noticing, but David Musgrave of the Developing for Dynamics GP blog released Support Debugging Tool Build 17 before Christmas; this is the build which added support for Microsoft Dynamics GP 2013.
He was back yesterday with a hotfix release to fix a couple of issues.
Partners can download SDT from PartnerSource (login required); if you’re a customer you’ll need to contact your partner to obtain it for you.
David has links to SDT for Microsoft Dynamics GP 10 and 2010 here.