Gabbard accusing Harris of obstruction doesn't make Harris guilty of obstruction. There is no legal progression between a statement by one party about the existence or veracity of a fact that could prove a crime by another party, and that other party and the veracity of the facts or the crime itself and forgone guilt, other than the statement by the first party itself could possibly be defamatory by publishing (making public statements) false facts.
By saying "If taken at face value (not saying it is actually true)", I think you're confusing obstruction with defamation.
If Harris did factually "block evidence", and it rises to the threshold of obstruction as determined by a prosecutor, then Gabbard isn't stating false facts and is not defaming Harris.
If Harris did not factually "block evidence", as determined by documents and evidence, then Gabbard could be stating false facts, but both are public figures, so the threshold of defamation is higher in that it requires actual malice by Gabbard knowing her statement is false or by reckless disregard for the truth.
To go a step further, to prosecute Harris, a prosecutor has to determine if Harris obstructed according to evidence, and charge her. Harris could have obstructed and not be charged for either political reasons or reasons regarding evidence, but that's a different question.