Who tries to belittle what Gratiano says in The Merchant of Venice? Do you think Gratiano's friends take him seriously? Why or why not?

Gratiano does quite a lot of talking in Shakespeare's The Merchant of Venice, and he often talks at great length, so we have plenty to choose from in terms of which one of his friends belittles what he says and fails to take him seriously.
In the first scene of the play, Gratiano holds forth for 25 lines on nothing in particular, but Gratiano is the first to make fun of himself about it:

LORENZO: Well, we will leave youthen till dinner-time.I must be one of these same dumb wise men,For Gratiano never lets me speak.
GRATIANO: Well, keep me company but two years more,Thou shalt not know the sound of thine own tongue. (1.1.109–114)

Gratiano isn't unaware that he talks a lot, and he's not unaware of the effect that his talking has on his friends.
Once Gratiano leaves the stage, Bassanio (supposedly one of Gratiano's friends) is talking to Antonio, another of Gratiano's "friends." Bassanio takes a passing shot at Gratiano:

BASSANIO: Gratiano speaks an infinite deal of nothing, morethan any man in all Venice: his reasons are as two grainsof wheat hid in two bushels of chaff; you shall seek allday ere you find them; and when you have them they arenot worth the search. (1.1.119–123)

Later in the play, Bassanio counsel Gratiano about his behavior:

BASSANIO: But hear thee, Gratiano;Thou art too wild, too rude and bold of voice;Parts, that become thee happily enough,And in such eyes as ours appear not faults;But where thou art not known, why, there they showSomething too liberal (2.2.167–172)

Bassanio isn't being entirely truthful, of course. His friends do find him wild, rude, and too loud.
It is Shylock, however, who best takes Gratiano to task for talking too much by belittling him and not taking him seriously:

GRATIANO: [to Shylock] . . . Can no prayers pierce thee?
SHYLOCK: No, none that thou hast wit enough to make.
GRATIANO: O, be thou damn'd, inexecrable dog!And for thy life let justice be accus'd.Thou almost mak'st me waver in my faith,To hold opinion with Pythagoras,That souls of animals infuse themselvesInto the trunks of men: thy currish spiritGovern'd a wolf, who, hang'd for human slaughter,Even from the gallows did his fell soul fleet,And, whilst thou lay'st in thy unhallow'd dam,Infus'd itself in thee; for thy desiresAre wolvish, bloody, sterved, and ravenous.
SHYLOCK: Till thou canst rail the seal from off my bond,Thou but offend'st thy lungs to speak so loud:Repair thy wit, good youth; or it will fallTo cureless ruin. (4.1.128–144)

Gratiano is uncharacteristically silent for quite some time after that exchange with Shylock. Gratiano's friends would do well to take a lesson from Shylock on how to control Gratiano's behavior.