Here's the Federal rules for Grand Juries: https://www.law.cornell.edu/rules/frcrmp/rule_6
According to my understanding of it (however under-educated that may be on law), AG Barr is correct when he says he cannot legally release the Grand Jury information to Congress. In my opinion, Congress, and specifically the Judiciary Committee, would have to petition the court that impaneled the grand jury for release of that information. If the petition were granted, Congress would have to maintain the secrecy of the grand jury information according to this rule.
The President's attempt to invoke Executive Privilege over the grand jury information in the Mueller report is an unprecedented extension of that privilege.
https://www.law.cornell.edu/wex/executive_privilege