That too goes through the Match. You can register for the Match by submitting an ERAS application and applying to at least one program. Since this is the end of the application cycle, you won't get any interviews, but you'll be eligible for the SOAP which usually has plenty of prelim GS programs.
Of course, the much bigger issue is, if you are able to secure a GS prelim year for next year, that will likely leave you time to interview at only a handful of programs (5 or 6) since you'll be working 80+ hours a week next year. You'd be much better off finding something else to do this coming year which would allow you maximum flexibility during interview season (Nov - Feb).