In September 2002 Magnus Gäfgen, a law student in Frankfurt, Germany, kidnapped 11-year-old Jakob von Metzler, the son of wealthy family with whom he had become acquainted, and demanded a ransom of one million euros. After the ransom was paid and Gäfgen arrested, Gäfgen refused to disclose the child's location. Would a Kantian support threatening Gäfgen or using force against him to get him to reveal where von Metzler was? Why or why not?

This is a question of ethics, and Kantian ethics is governed by what the philosopher called the "categorical imperative." Kant expressed this as a variation on the Golden Rule, namely "act according to the maxim that you would wish all other rational people to follow, as if it were a universal law." The important thing to understand is that the consequences of any action should not be taken into account but rather whether the action is consistent with the categorical imperative.
This question is a good one because it raises the question of whether a desirable end—locating the young man who had been kidnapped—justifies an action that we would not want to become a universal law. In short, a Kantian would oppose torture. They would view the employment of torture on a situational basis as irrational and immoral, a violation of the categorical imperative. To Kantians, morality by its very nature can not be situational, and cannot depend on the potential consequences of an action, however desirable they may be.
https://plato.stanford.edu/entries/kant-moral/

Torture can be defined as "the officially sanctioned infliction of intense suffering, aimed at forcing someone to do or say something against his or her will." (Rodley, 2000:7)
In his absolutist ethical theory known as Kantian ethics, Kant argues that a person is good or bad depending on what motivates his or her actions and not on the goodness of the consequences of those actions. This means that someone can have moral worth (be a good person) if he or she is motivated by morality.
Kant has no concern for the consequences of actions, only what motivates them. He basically states that if two people act for the right (moral) reasons, they are both morally worthy, even if the actions of one of the two may lead him or her to bad outcomes. According to Kant a person is only good if he or she does his or her duty because it is a duty.  It does not matter if they do or do not enjoy doing their duty; their moral good lies in the fact that they want to do it. The essence of Kant's argument, in this regard, is that for one to be a good person one has to be good for goodness's sake. In terms of this approach, therefore, any Kantian absolutist would definitely have opposed threats of torture or violence against Magnus Gäfgen. Kantian absolutists do not care about the consequences of not torturing him. They only believe in doing what is morally worthy and the most ethical thing to do. Consequences to Kantians are insignificant, whether they are good or bad.
In the above scenario, although it appears as if the deputy police chief of Frankfurt at the time, Wolfgang Daschner, acted for the greater good by threatening Magnus, his action would be deemed morally unacceptable by Kantians. Although the consequences of his actions might ensure a good outcome in that the police might find Jakob von Metzler, the threat of torture against a person is deemed morally wrong by Kantians. It is the act itself which should be morally right and not the resultant consequences of such an act.