She told the auditionees: ' hope all of you have a great day and remember to go all out! You've got one shot - one chance, so make sure you give it your all.
"Be yourself, the only thing you can offer is originality to the world. There's a lot of great singers and dancers but you have to do what makes you unique. Good luck and make Miami proud!'
Gloria's impromptu appearance even shocked show boss Simon Cowell.
In a conference call with various journalists, the music mogul said: ' had no idea she (Gloria) was going to be there but apparently she did a fantastic job . She gave everyone a lot of support and encouragement. Not a lot of people would bother to do that."
During the conversation, Simon also confirmed Black Eyed Peas star Fergie had been considered for a spot on the judging panel, only revealing her name had been "put forward".
Simon recently revealed he is finding it incredibly tough deciding who should join him and confirmed judge L.A. Reid on the panel.
He said: "The truth is that the reason we haven't announced the panel yet is that nobody can agree. New people keep coming along every week who we want to see.'
Nicole Scherzinger, Jessica Simpson and Katy Perry are also believed to be among the stars in line to judge the competitors.
The US 'X Factor' is set to hit screens this autumn on Fox.
