It is hearsay. But that doesn't mean it's not credible. Nor inadmissible. There is no prohibition against hearsay testimony in the inquiry that I'm aware of, and it would be non-sensical to exclude it from an investigation. That's where leads come from. Obviously, it holds little value without corroboration from the staffer and/or Sondland. Technically Trump but... Yeah don't expect to hear him testify or do so truthfully if he did. Anyway, hearsay is absolutely admissible for federal grand jury testimony. I would expect it to be excluded from a Senate trial.
Anyway, I wish there were a recourse against overt lies, for example the assertion that Schiff knows and met with the whistleblower. The reality of course being that one of his staffers advised him on filing a whistleblower complaint after his criminal referral to the CIA lawyer was killed by Barr. I've seen it crafted on the radio to the extent that Schiff and his lawyers wrote the report itself, and because the report was filed a month after the call, the concern was therefore manufactured as a means of outing Trump. Nevermind the impetus for bringing the information to Congress was a person implicated in the report itself as being involved getting to decide that the issue was non-criminal.