I honestly would think Gabbard might hurt Trump more than the Democrat, whoever it is.
Bloomberg is the opposite. Bloomberg getting in to the race (to try to capture the middle if the Dems nominate Warren) probably splits the Dem vote more than the Republicans. Ensures Trump gets a plurality everywhere he needs it.
Objectively speaking, looking at a general election, I think she's the only Democrat who could possibly beat Trump in 2020.
Don't get me wrong... I don't agree with her on very much, and she'd not get my vote. But she'd get a lot of votes from those who are looking for a sane alternative to the President.
All the others have gone so far out on the leftwing limb that they can't possibly get back to the center.