Rome w/o Gaul?

The thread a few days ago about a larger Roman Empire in the West (taking over either Germany or Persia) got me thinking: what about a SMALLER (but maybe longer-lasting) empire? Certainly, one that still dominates Italy; otherwise, they're just not so important to history. And I still want them to win the Punic Wars too (otherwise, it's more like "what about a huge Carthaginian Empire?"), giving them Spain and a lot of North Africa, at least. Also, I'll give them Greece at least (otherwise, it's too culturally different from OTL Rome for my taste), and maybe even their other Eastern conquests (I'd be interested to hear possibilities in that area, but I think we at least need Egypt for its wealth).
But Gaul, it seems to me, is pretty easily avoided: we just kill off Julius Caesar before most of the events of the Bellum Gallicum, and replace him with a more cautious or less militarily gifted governor. And we can add that Augustus*, as in OTL, has a will limiting the boundaries of the empire to the current ones, including the boundaries of the original province of Roman Gaul (will that hold? Is Gaul just too tempting, in the long run?).
Does leaving Gaul alone insulate Rome at all from the German migrations later? Does it leave fewer opportunities for 3d-c. generals to win glory and declare themselves emperor? Is the East-West split less inevitable with less territory in the West? How much does Gaul matter, in the end?

*I mean, of course, Sextus Pompeius Augustus, first emperor of Rome.
 
Why not we use mailinutile2's idea about a 'smaller' Roman Empire...?
Rome only control Italy up to the Alps, Narbonensis, Baetica, Africa, Cyrenaica, Dalmatia, Achaia, and Asia Minor up to Taurus mountains...?
Because it's kinda strange for me that Rome never conquered Gaul but still conquered the entire Hispania...

Btw, about your questions:
1. Assuming the Celts remained controlling Gaul, Germanic invasions to Gaul may not happened. AFAIK it was because of the withdrawal of Roman forces from Gaul to defending Italy, that causing Germanic tribes easily occupied Gaul. So, if the Celts developed a huge population base in Gaul, and even built an empire that stretched from Britain to the Alps...who knows...?
2. About east-west split, in OTL it was caused mainly by increased threats of both Germanic tribes and Sassanids, and that the empire was too large to be governed by one people...so in 'smaller' Roman Empire, there would be no split at all...
3. If Romans never conquered Gaul, Celtic language and culture lasting longer, maybe...?
4. About 3rd century crisis...yeah, some people in this board said that smaller Roman Empire means smaller problem...however, in the long run, the Romans may have much troubles, as they never acquired the 'easily' defensible Rhine (and assuming we use mailinutile2's scenario, Danube and Euphrates as well...)
5. And with no Gaul = less population = less taxes...and less population also = less army...
 
I think a more likely border is Danube to Switzerland, Down the Rhone Valley, Along the Narabo Coast to Hispaniola, then a border in the Pyrenees. North Africa can be the Same to Upper Egypt, then along the Zagros to the Caspian, then back through the Caucuses to the Black Sea.
 
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