The No.1 Gru Cosplayer is the online nickname of Marco Buzzari. He is also the president of the name brand fashion company Gucci, which is a popular brand among millennials and some centennials, as it is a status symbol of wealth.
The nickname originated when a video surfaced online of when Marco visited Parson's School of Design, and what seems to be after a seminar of some sort, Mr. Buzzari is standing and taking pictures with the students, and one student proclaims,
"You look like Gru!"
He seems to be confused, and does not understand in the end.
Friend: Hey, I heard you finally met The No.1 Gru Cosplayer! How was it?
OP: It was awesome!