I'm not sure Gratian as sole emperor in the West would have been this a net good, but on the other hand the triparte division of Romania was no longer viable either.
I think you'd need Valentinian living longer than IOTL and prevent his elder son (as well as his cadet) being too much under the influence of their advisors : I don't think these really did a number on Gratian, but he was really dependent from them and the army structure (which meant Frankish commander-in-chief), which in turn pissed a lot of people in a period of great military need.
I agree, however, that with the right kind of upbringing, Gratian could have turned his potential into real skills. Furthermore preventing the civil war (although it's far from obvious Theodosius wouldn't have pulled something similar against Gratian ITTL, let's assume it's the case) would prevent the political growing importance of foederati and mercenaries, or rather how it happened than butterflying it away. However, I think it would made Franko-Roman dominance over military command lasting longer ITTL, due to a relatively lesser importance of Gothic element in western Romania, meaning possibly no Stilicho but Arbogast or its replacement.
Would have it be enough to prevent the crisis in WRE? It really depends how it unfolds : if Theodosius is fine with Gratian, if Gratian manages to balance ambitions and expectations (and not play favourites with specific foederati or mercenaries as IOTL). But preventing the tensions with ERE would allow a better hold on both Illyricum (the traditional soft underbelly of Romania, and a main concern for Italy) and Rhineland.