Can the GACC California guarantee that I will receive my DS-2019?

No, there is no organization that can provide you with such a guarantee. The success of your  DS-2019 request depends on whether your selected internship or training host organization and yourself are suitable for the J-1 visa, and whether all necessary information is submitted in time. Visa agencies issuing the DS-2019 must act strictly in accordance with the criteria and guidelines provided by the U.S. Department of State. Reasons for a DS-2019 rejection vary greatly. Possible reasons may be: Your selected company does not cooperate with the visa agency; your internship/training mission is not relevant to your studies; you seem to have insufficient funds to pay for your stay in the U.S.;it appears you do not intend to go back to your home country after you completed your internship/training with the U.S. company.