@MonicaH, on a practical level, there's nothing you can do.
GMB support will say "take a number" and not do anything. Neither "suggest an edit" nor the redressal form deals with reviews. Even if you got a human to look at the corpus of reviews, the question would become, "So which specific reviews aren't legit?" and you couldn't answer, because any review may or may not have been incentivized. As you say, the competitor was smart not to announce the raffle. Google won't throw all of the reviews out.
So that leaves flagging each review, one at a time, in the hopes of thinning the herd. That's not real sustainable, especially because Google already let the reviews go up.
The good news is I'd say this competitor isn't
that smart. If they were formidable they wouldn't go cheap by entering would-be reviewers into a drawing, but would simply give every reviewer the $50 card. The raffle isn't too enticing, so most of the people who bother at all will just dash off their reviews. Eventually all the reviews look fake or forced. The competitor may or may not even get a higher review count than they would if they just asked customers for a favor, and the reviews won't have the detail/keywords that great reviews have, and the reviews won't be persuasive.
It's counterproductive for the competitor, so I wouldn't get in their way.