How is Gatsby different from his guests? What are the rumors told about Gatsby? What roles does Meyer Wolfsheim play in the novel?

Here you have given a three-part question about The Great Gatsby.  Let's take each part in turn.

First, Gatsby is first very different from his guests because he is never seen as his parties.  Everyone else who attended, or who "went there" according to Nick, went there specifically TO be seen. 

In his blue gardens men and girls came and went like moths among the whisperings and the champagne and the stars.

Gatsby didn't care to be seen.  All Gatsby cared about was enticing Daisy to come to the parties, but she never came.  Gatsby is also of the upper class, unlike many of his guests.  For example, when one woman ruins a dress at Gatsby's party, he makes sure to order her a new one, a BETTER one.  It is also significant here to mention that Gatsby is one of the "new rich" and as such lives in "West Egg."  (This is in comparison with the middle class who live closer to the city, in the Valley of the Ashes in Queens, and also in comparison with the "old rich" who live in "East Egg.")  Here let me provide one more important quote about Gatsby's parties:

Laughter is easier minute by minute, spilled with prodigality, tipped out at a cheerful word. The groups change more swiftly, swell with new arrivals, dissolve and form in the same breath; already there are wanderers, confident girls who weave here and there among the stouter and the more stable, become for a sharp, joyous moment the centre of a group, and then, excited with triumph, glide on through the sea-change of faces and voices an colour under the constantly changing light.

Second, there are many rumors about Gatsby that are started at his parties.  They are started precisely because of the answer to the first question:  Gatsby is never seen and can never defend himself.  There is a rumor that he is "an Oxford man."  There is a rumor that "he killed a man once."  There are MANY rumors, some of them true (like the one that he was "in the war"), some of them half-true, some of them false, and some of them that might become true.  (After all, Gatsby does play a role in killing Myrtle near the end of the book, even though it was Daisy driving.)

Third, Meyer Wolfsheim plays an interesting role in the novel. He proclaims himself to be Gatsby's "friend," and yet refuses to come to Gatsby's funeral, even when Nick begs him to do so. Meyer Wolfsheim is Nick's first real clue that Gatsby isn't all that he says he is.  Meyer Wolfsheim even has cufflinks made of "the finest specimens" of human molars.  (Who has cufflinks of human molars?!?)  Nick is with Gatsby one day as he meets with Meyer Wolfsheim.  Meyer Wolfsheim believes Nick is looking for "a business gonnection," but Nick is most certainly NOT.  Nick recoils precisely because Meyer Wolfsheim seems to be involved in something shady.  We learn later that the two are involved in an intense bootlegging scheme.