If Congress has the authority to physically compel people to appear before it at any time for any reason, such a statute would be an exercise of its authority. A mere statute, however, could not confer upon Congress any authority which it did not already have.
In any case, I would suggest that whether or not Congress has the authority to compel people to testify, it should have the authority to regard the giving of testimony as an essential job requirement for federal employees.
Congress has the clear constitutional authority to impeach federal judges. Gingrich's rationale was to subpoena a judge to explain his rationale prior to initiating an impeachment hearing. The Subpoena is a courtesy to the judge.
Congress has the power to subpoena a judge the same as they can subpoena members of the executive. But if a judge wants to ignore a congressional subpoena, then they can answer to the impeachment trial.