Either your Rome will expand to conquer the adjacent regions, or if it is as week as it needs to be in the first place for this to happen then it will fall and succumb to Hellenic cultural dominion. The Greeks are at a geographical disadvantage for holding on to their colonies after a Roman entity forms. Also with any PoD before the Moors Lusitania would have no reason to become independent from Spain, so you probably would have no Portugal.
In fact, Greek armies invaded Italy several times. Not to mention the high Greek population centers in the South such as Syracuse, Taras, Neapolis, and Rhegion. It seems incredibly improbable that Greeks would conquer Gaul and Iberia without using Italy as a stepping stone.
But we still run into plausibility.
The only strong, western-looking Hellenistic or Hellenic state that could fit the bill is Epirus. Wank the hell out of Pyrrhus, you have Italy, Sicily, and Carthage under his control in the West as well as Epirus and Macedonia in the East. From there, if such an empire can sustain itself, expansion might come at the expense of Northern tribes, but is that likely at all? Massalia might hope to ally itself with this Pyrrhic Empire like it did with Rome for protection from the Gauls, and if some Epirote general or prince conquers in a way analogous to the Barcas, you can see some of Hispania conquered.
But Hellenistic armies function very differently from Roman armies, and they won't be able to defeat the Gauls, who happen to be the people that made up much of their armies as mercenaries.