What would have happened had the Gauls successfully unified and

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Let's say they fended off the Roman invasion, what happens next? I'd assume Caesar would be tried for not only waging his illegal war but losing it and humiliating Rome in the process but how would the rest of Rome react? Would they still want vengeance against the people who sacked their capital hundreds of years ago or would they prefer peace with the Celts?
How would this affect Rome in the long run? On the one hand they don't have to spend so much money and manpower civilising their northern neighbours but on the other hand they'd miss out on the soldiers and materials gotten from them.
Where would they spend time if not for western Europe now? Advance deeper into the Balkans? Arabia? Persia? Explore deeper into Africa?

And what of the Gauls? Would they try invading Iberia or would they not want to try their luck against Rome again? Push east into Germanic tribes? Or take Britain? Would their common Celtic culture make it easier for them to assimilate other Celitc areas?