Slow Flight for CFI/Commercial Pilot - Should the gear be up or down?
Slow flight in training and testing has been the subject of much discussion and reevaluation in recent years.  The philosophy of how to train and demonstrate slow flight has changed a bit from years ago. 

In specific answer to your question regarding whether or not the landing gear should be extended or not, here is an excerpt from FAA H-8083-3B [2016], page 4-4: Airplane Flying Handbook

(emphasis mine)


  Slow flight is typically performed and evaluated in the
  landing configuration. Therefore, both the landing gear
  and the flaps should be extended to the landing position.
  It is recommended the prescribed before-landing checks
  be completed to configure the airplane. The extension of
  gear and flaps typically occurs once cruise power has been
  reduced and at appropriate airspeeds to ensure limitations
  for extending those devices are not exceeded. Practicing this
  maneuver in other configurations, such as a clean or takeoff
  configuration, is also good training and may be evaluated
  on the practical test.


Here is the current flight test guidance for a Commercial Pilot applicant: "Airman Certification Standards" (ACS).  Under Area of Operation VII., "Slow Flight and Stalls," Skills: CA.VII.A.S4: - Commercial Pilot - Airplane, Airman Certification Standards - June 2017

(emphasis mine)


  Accomplish coordinated straight and level flight, turns, climbs, and descents with landing gear and flap configurations specified by the evaluator without a stall warning (e.g., aircraft 
  buffet, stall horn, etc.). 


Also, here is an FAA SAFO (Safety Alert for Operations) entitled "Maneuvering During Slow Flight in an Airplane" (8/30/2016): SAFO 16010