A friend who holds a senior foreign-policy position in an EU government was absolutely horrified at the prospect of a subpoena to Gross. No admirer of Trump’s, this person felt such a subpoena would create a new precedent that would shadow all future confidential presidential conversations with non-English-speaking heads of government, allied as well as adversarial.
The Trump administration would surely raise an executive privilege objection. In the 1974 case of U.S. v. Nixon, the Supreme Court rejected an absolute claim of internal privilege within the executive branch. But it acknowledged “the valid need for protection of communications between high Government officials and those who advise and assist them in the performance of their manifold duties.” The court stressed that this privilege was at its strongest in military and diplomatic affairs, which would presumably describe the Helsinki meeting.
There is a non-trivial possibility that Gross might refuse to testify.