Hold up. That's not what Appian says. Appian says:

ἐφ᾽ ᾧ δὴ μάλιστα ἡ βουλὴ διαταραχθεῖσα τοὺς ὑπάτους ἐκέλευσε προγράψαι μηδένα τῶν οὐ φερόντων ψῆφον ἐπιδημεῖν τῇ πόλει μηδὲ προσπελάζειν ἀπὸ τεσσαράκοντα σταδίων παρὰ τὴν ἐσομένην περὶ τῶνδε τῶν νόμων χειροτονίαν

The senate being much disturbed by this [that C. Gracchus proposed the extension of suffrage to the Italians and that he summoned them to the city during the vote on his Italian bill to demand their voting rights], and it ordered the consuls to post publicly that no one not possessing suffrage would be in the city or approach within forty stades of it while there was a vote on these bills

Appian says that Italians were not allowed into the city during the vote on these specific Gracchan bills. They didn't prevent Italians from entering the city at other times, just for this one occasion. That the influx of Italians and rural voters was considered a threat is quite clear: Plutarch mentions that during C. Gracchus' bid for the tribunate so many Italians and rural voters entered the city that they could not be housed in the Campus Martius and had to climb adjacent rooftops to shout down their votes. Interestingly, Plutarch mentions a slightly different version of the edict to remove Italians from the city:

ὄχλου δὲ πανταχόθεν αὐτῷ συνιόντος ἔπεισεν ἡ βουλὴ τὸν ὕπατον Φάννιον ἐκβαλεῖν τοὺς ἄλλους πλὴν Ῥωμαίων ἅπαντας, [2] γενομένου δὲ κηρύγματος ἀήθους καὶ ἀλλοκότου, μηδένα τῶν συμμάχων μηδὲ τῶν φίλων ἐν Ῥώμῃ φανῆναι περὶ τὰς ἡμέρας ἐκείνας, ἀντεξέθηκεν ὁ Γάϊος διάγραμμα κατηγορῶν τοῦ ὑπάτου, καὶ τοῖς συμμάχοις , ἂν μένωσι, βοηθήσειν ἐπαγγελλόμενος, οὐ μὴν ἐβοήθησεν, ἀλλὰ ὁρῶν ἕνα τῶν ξένων αὐτοῦ καὶ συνήθων ἑλκόμενον ὑπὸ τῶν ὑπηρετῶν τῶν τοῦ Φαννίου, παρῆλθε καὶ οὐ προσήμυνεν, εἴτε τὴν ἰσχὺν ἐπιλείπουσαν ἤδη δεδιὼς ἐλέγχειν, εἴτε μὴ βουλόμενος, ὡς ἔλεγεν, ἁψιμαχίας αὐτὸς καὶ συμπλοκῆς ἀρχὰς ζητοῦσι τοῖς ἐχθροῖς παρασχεῖν.

But when a crowd came to him from all parts [of Italy] the senate persuaded the consul Fannius to throw out all others but Romans. And when there came about an unprecedented and unusual edict that no one of the friends and allies should appear in Rome during those days [i.e. the day of the vote and those days preceding it] Gaius issued an edict attacking the consul and promising the allies, should they remain in the city, that he would help them. But he did not help them, but seeing one of his friends [the Greek here says ἕνα τῶν ξένων αὐτοῦ, "one of his guest-friends," using the familiar Greek term ξένος, a stranger tied to an individual with bonds of mutual hospitality. Probably Plutarch means one of C. Gracchus' foreign clients, of whom his branch of the gens Sempronia had many throughout Italy] and comrades dragged along by the apparitores [probably specifically the lictors, but Greek is unclear about these minor staff offices] of Fannius, he passed by and did not help him, either fearing to prove that his power was already weakening or not wishing, as he said, to provide the beginning of fighting and struggles to his enemies, who were seeking an excuse

In any case, C. Gracchus' vote failed spectacularly, in the same way that his brother's bid for reelection to the tribunate failed. In Ti.'s case Appian tells us the rural voters and Italians failed to come to the city in large enough numbers to make any difference, since the elections took place during the harvest, which they could not miss. In the case of C. Gracchus, the combination of the ejection of Italians (probably neither very well enforced nor especially effective; see Mouritsen's discussion of the demographics at the vote and the general nonexistence of "identity control" at the ballot in Plebs and Politics. In any case C. Gracchus' non-citizen Italians could only indirectly influence the vote, and rural citizen voters could still enter the city just fine) and Livius Drusus' counter bill proposing the establishment of twelve colonies spelled Gracchus' doom--colonial foundations appear generally to have been more widely favored by the urban plebs than land distributions, which benefited rural dwellers much more, generally did. Moreover, says Appian, Drusus vetoed C. Gracchus' motion. Presumably without the presence of enough rural voters to swing the vote and with clear support for Drusus' colonial bill among the urban plebs Gracchus would not have had an easy time abrogating Drusus' magistracy, had he opted to follow his brother's precedent when Ti. abrogated Octavius' tribunate (quite possibly by using the numerical superiority of the rural tribes to bully the urban tribes into submission, provided that we assume urban support for Octavius). C. Gracchus, in any case, received the commission to establish a colony at Carthage under the lex Rubria--122 was apparently an active year among tribunes vying to establish agrarian bills and colonial foundations, and it's possible that the apparent prevalence of colonial bills might indicate something about C. Gracchus' tribunician program (I sense the possibility of a new project for myself?). Lintott mentions the provisions of the lex Rubria, and it's rather odd. Apart from the resettlement of six thousand colonists (more than originally provided by the law), the territory around the colony was also sold by the quaestors to Italians, Latins, and Roman citizens. Lintott suggests that this oddity may have been authored by C. Gracchus, either as a compromise with Drusus and Rubrius or by a separate Lex Sempronia that may be mentioned in the lex agraria of 111 that confirmed much of the land legislation of the second century.