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.
If a judge who happens to e.g. in Texas decides that he'd rather stay in Texas and forfeit his job than travel to D.C., what authority would Congress have to do anything more than declare his job forfeit? If Congress has arbitrary power to subpoena people who are not employees of the federal government, what would prevent them from using such power arbitrarily and capriciously so to impose hardship upon people who do things Congress doesn't like?