So is it mandatory that in a given turn that a minion must chose one of the two actions per steadfast champion? (does the minino either have to assign to gandalf or let the free peoples player heal gandalf?)
No, each minion automatically gains the ability to be assigned to Gandalf while the condition is in play.
Completely separate from that is the fact that the Free Peoples player may discard the condition during the regroup phase to heal Gandalf up.
The idea is that by the Shadow player being able to assign any minion to Gandalf, the Shadow player has a better chance of killing Gandalf (thereby preventing the regroup action from occuring).