Suppose Caesar avoids the blades of the Liberators and commences his proposed invasion of Parthia. As far as I understand it the various Celtic and Germanic tribes living in Gaul did not have very organised armies. To satisfy personal honour they often attack in a horde, so that individual warriors would have a chance to win glory in one-on-one combat, allowing the tight ranks of Romans to defeat them time and again.
While the Gaulish armies (and up to a point, Germanic ones) certainly were not made up of
soldiers, that is all the technicity, discipline, commandment skills, logistics, etc. that characterized the quasi-professional "Marian" legion, that's still a simplistic take on late Latenian warfare.
We know, mostly from Caesar's commentaries (but also from other Greek and Roman historians) that Gauls were able to manoeuvre and hold formations : often a tight-packed phalanx (as was the rule for non-professional classical armies), but also tortoises, envelopment, using gaps in line to move in and out archery (in a very similar way as Romans did), lines formed by warriors of various quality, etc. Nothing particularly comparable to Hellenistic or republican Roman armies, but far from a primitivist one-on-one warfare.
There's a really interesting argument about how late independent Gaul actually went trough a military revolution (and a militarisation of society trough clientelisation and "expension" of warfare on urban classes), along the centralization of the regional petty-states, including mobilisation capacities.
The main problem of Gaulish armies were, well, that they weren't quasi-professional : training, discipline, cohesion, equipment, logistics, etc. was very likely all over the place (even more so as the size of these armies likely expanded) with plethora of war leaders whose hierarchization was likely not very obvious past an high command elected/appointed by a people or coalition : the troops themselves were certainly divided up between groups of very different technicity from a warring elite to a lot of levies or homeguard with everything in between; whereas Caesar's "Marian" legion was extremely cohesive, under a clear and cohesive military and political hierarchy, with a better tactical and technical make-up.
(We otherwise know that Gauls served under Caesar during the Gallic and Civil wars as a skilled cavalry but as well as archers, the Ruten archers being the only ones being specifically named alongside the famed Cretan archers, hinting at a good technicity.)
Note that even in these conditions, the last Gaulish coalitions tended to be more threatening precisely because Gaulish commanders started to adapt : Vercingetorix' coalition thus went really close to defeat Caesar not by sheer numbers, but because of strategical and tactical aggiornamento (very likely born out of a lot of Gauls being Caesarian auxiliaries) resorting to hammer-and-anvil strategy, supply denial, etc.
Caesar genuinely had to display and tune political and military skills during the campaigns that were far enough from being a cakewalk he resorted to not-so-common sieges (such as the double fortification) and brutal practices that even Roman felt were veering on war crimes to deal with the situation (which, giving how Romans were comfortable with the right of the victors, is somewhat remarkable). I wouldn't say that a Caesarian defeat at the end of Parthians would have come from him being a strategic fraud confronted to an actually real army.
Relatedly, you could make the argument that the Parthian army itself wouldn't really be considered as professional either, at the least not to the point the late Republican army was.
A non-standing army (which arguably the late Marian legion still *technically* was itself) made up of levied fighters ranging from highly skilled nobles or guards to fairly secondary client infantry; whose logistic capacities were limited by the length of service and the "feudal" organization of the army (and thus often leading Parthians to resort to an attrition war leading Romans further from their bases), a rather complex and "decentralized" military command.
This doesn't diminish the very real military capacities of the Parthian armies as they existed IOTL but, there as well, I don't think that would necessarily mean Caesar would have a field day or, conversely, be outskilled.
