Passive movements aren't used for alleviating spasticity - they simply don't. What they do as Gawaine mentioned is maintain range (hopefully - rather than dealing with it retroactively). I believe that there is quite a lot of literature on stretching retroactively - most of it controversial and based around the question of how long you need to do it for it to be effective - try using stretching as one of the search words.
As for using them prior to gait retraining I'm not sure quite what you're asking - in the same session, just before gait training or in the build up toward being able to start gait training?