But who they were actually watching was Tierney Allen, a Lady Gaga impersonator who performs regularly in Legends in Concert, a tribute review at The Tropicana that also features Freddie Mercury and Pat Benatar lookalike/soundalikes.
Contacted by The Hollywood Reporter, Allen says she had no intention of misleading the audience into thinking she was really Gaga.
"What happened is I was booked to perform at a private event for Women of Global Change. We always tell the clients to make sure the guests know I'm not the real thing," Allen says.
The confusion began when, at the end of her performance, the event emcee, Aurea McGarry, told the crowd that Allen was not an impersonator but was in fact the real thing.
"I thought, 'Oh, that's not good,'" recalls Allen. "I immediately started sweating. I must have looked like a deer in the headlights."
Allen has been mistaken for the artist before — she was once swarmed by fans while visiting the Park MGM in full Gaga regalia while the "Poker Face" singer was performing there — and decided to just "go with it" until she could get off the stage.
Allen says she had assumed McGarry knew very well that she was a fake — but thought the emcee got swept up in the moment and wanted to give the audience a thrill.
But lawyers for McGarry tell THR that she was totally in the dark, as well, because an organizer with Women For Global Change told her in confidence before the event that "Lady Gaga would be making an appearance and perform at the conference."
"Unbeknownst to our client," McGarry's lawyers write, "it was not Lady Gaga who appeared and performed."
The resulting confusion led to fans rushing the stage, swarming the impersonator and snapping photos of her.
The ruse grew even thornier when Allen was then enlisted to help in a charity auction of a framed A Star Is Born poster autographed by the film's two stars, Lady Gaga and Bradley Cooper. That piece of memorabilia was authentic. But when the bidding began, the crowd was encouraged to spend more in the presence of Gaga.
"The actual Lady Gaga is holding it!" McGarry told the crowd of the poster. A bidding war erupted between two women; it ultimately went for $3,000. Lawyers for McGarry maintain she believed the woman holding up the framed poster to be the real Gaga.
When Allen finally left the event, her last words to an event organizer were, "I really hope I don't upset Gaga's people."
Among those totally fooled was The Strat itself, which issued its press release touting Gaga's appearance at 3:09 p.m. PT on Friday. The release included two photos of Allen performing onstage.
Fifteen minutes later the hotel issued a retraction.
"An item was sent from our office earlier today regarding an event with Lady Gaga," it reads. "Unfortunately, new information has come to light that suggests it may not have been Lady Gaga, but actually a tribute artist."
Contacted via email by THR, a rep for the hotel responds, "There has been some miscommunication between several parties. We will keep you posted if we hear anything." Reps for Women's Rights for Global Change did not respond to repeated requests for comment.
Nov. 11, 10:41 a.m. PST Updated to include statement from McGarry's lawyers.
This article originally appeared on The Hollywood Reporter.