The expedition on Rome seems to have been a raid (would it be only because a large part of raiders didn't settle in Italy eventually) and/or, if you trust Roman historians, an expedition to prevent Romans to help Etruscean and northern Italic peoples to intervene on Northern Italy.
Paduan basin seems to have interested them more, for obvious reasons (more wealth).
You'd need more Celts (rather than Gauls*, many Celts in Italy being issued from diverse fractions whom some settled in Gaul, other didn't or did only later. It's as well possible that we're talking of two distinct peoples with roughly the same name) and more issues in others regions to motivate them to do so.
Now, I'm not too sure what you call semi-united. Each people worked his way idependently. It could means alliance with others peoples (Celts or not), but you can't really tell of a Celtic unity (even if really reduced) then.
Assuming they take not only the southern part of Po's basin, but as well Tiber's (and again, that would ask for a far more important Celtic presence, which would make an earlier PoD necessary, and plausibility may not be on your side depending which you would choose) : it's probable these Celts would be more or less Italicized.
Not by Romans, obviously, but probably trough contact or conquest with peoples such as Samnits or Etrusceans. It was already the case for a good part of southern Paduan basin celtic peoples, and I don't see why the absence of Rome as an Italic political power would change that.
Eventually, giving their position, limited numbers (even by reinforcing them), I'd say that Samnits or other highlands people would eventually spread on coastal plains : Roma as well as Capua.
*Strictly speaking : Romans called Celts "Galli", but we tend nowadays to reserve this name to populations living in the region named "Gaul" by Caesar. With the twist that not all Gauls were Celts.
