I must strongly protest the other poster on here, whose source consists of his own blog, alongside what are essentially pieces of historical fiction. His information is misleading at best, and outright mistaken in many places. See [this]9https://www.reddit.com/r/AskHistorians/comments/9x7pwj/how_did_the_roman_republic_system_of_government/) recent thread for a fuller discussion.

In particular the other poster claims that the agrarian law, which he describes oddly as a "redistribution of large estates" (which misunderstands the nature of the ager publicus and does not take into account recent research by Roselaar on the Gracchan privatization of the ager publicus), was "anathema to most members of the senatorial class." He provides no justification, and will find none. Stockton showed pretty conclusively that Gracchus did not receive significant senatorial opposition until he proposed his law distributing the fortune of Attalus, which traditionally should have been the senate's right. We know of many senatorial allies of Gracchus at this point working in favor of the lex agraria, including the princeps senatus himself. Of senatorial opponents at this stage we know none. Stockton argued persuasively on the basis especially of Plutarch that Gracchus' revised agrarian bill had the general support of the senate, and that Octavius' veto was unexpected.

The poster claims that following in the veto, in according with "usual practice, it was sent to the Senate for adjudication." That's not correct. There was nothing in the mos maiorum that prescribed senatorial review of vetoed bills. A vetoed bill usually just disappeared. Nor was Gracchus' reintroduction of the bill--which, as I mentioned above, Stockton showed was likely supported in the senate after it was proposed in its revised state--controversial at all. David and others have showed pretty much without any doubt that Gracchus had excellent precedents for his introduction of the bill without senatorial approval. In any case, there doesn't seem to be a good case that the senate refused the bill.

It should be noted that though Gracchus' calling on the tribal assembly to abrogate Octavius' magistracy was unprecedented, Octavius' use of the veto was as well. David points out that Octavius is the first tribune known to have vetoed the rogatio--the mere reading of a bill--before the assembly. This seems to play very well into the argument that Plutarch presents Gracchus as giving, namely that Octavius unconstitutionally removed the power of elections from the assembly.

Nor do we have evidence for what the senate thought about the abrogation of Octavius' magistracy (which, to answer your question, was done by a vote of the tribal assembly, which was called up by tribes to vote) at this stage. We do not have the opinion of a single opposed senator given to us in the sources before the distribution of Attalus' property. After that point Plutarch has Metellus oppose Gracchus, but on the grounds that Gracchus' bill distributing Attalus' property was against the moral austerity of his ancestors. Plutarch likewise at this point includes Annius, who was invited by Gracchus to a contio defending the abrogation of Octavius' magistracy. Annius reversed the contio by challenging whether Gracchus would allow him to call Gracchus' tribunician colleagues to witness without abrogating their magistracies. But this took place after the bill distributing Attalus' property, and the near-total absence of any real resistance to the agrarian bill other than Octavius is noteworthy, and commented on by Stockton.