I recommend conceding the Po valley (Mediolandum etc) to the Julii and setting up two all cavalry armies around Massilia. As the pre-Marius Roman infantry (even Triarii) can't handle cav that well, you can fight a holding action with the Julii in southern Gaul while sending your better infantry into either Spain or Britannia. Don't try to fight against all three of your neighbors simultanously, though, as you will be sandwiched by Spainards and Julii. It becomes a whole differant ballgame when the Marian reforms kick in. As I never got that far with Gaul (re-installs, mods, life getting in the way) I really can't tell you how to take on the elite legions with Gaul.