It's something that pretty much every movie & film has gotten wrong, to the point that it's commonly accepted that the thumbs up means "live". There's a good chance that Gladiator did it incorrectly on purpose, because doing it the right way would just confuse people at this point, because we've had nearly 100 years of cinema teaching us thumbs up = good, thumbs down = bad. The real Romans never used a thumbs down.