I don't know anything about it, but was the metallurgical knowledge of the Romans good enough to be able to make guns?
Yes, it was. Ever gotten a close look at a gladius? Beautiful weapon.
The problem is two-fold. First, the gunpowder itself. Black powder is touchy stuff at best; manufacturing, storing, and transporting it would be problematic. Add to that the fact that the powder needed for cartridges is a different mixture than that used for priming, and the powder used for artillery is different than either of the above, and it will be quite a headache until they get it sorted out.
Secondly, the amount of iron and steel required for the legion's weapons and armor is roughly doubled, which means that the manufacture of iron and steel will have to be hugely increased, which means less workmen available for other uses. The economic butterflies will be huge.
Okay, assume the Romans have gunpowder weapons. How will they use them?
At first, probably much like the Highlanders, who simply supplemented their swords and shields with pistols and muskets. They fired their weapons to soften up the enemy before closing in with their swords. Was that effective? Definitely. And it would suit the Romans perfectly. Better, in fact, since the Romans would use volley fire, which would be devastating at the short ranges they're accustomed to.
The New Model style of army really doesn't suit the Roman mindset; the legions are all about offense and mobility. Muskets would be used by light troops, including cavalry, and supporting troops for the purpose of softening up the enemy prior to closing, or on the rare occasions when Romans defended prepared positions.
So, here's our legionary, circa 100 A.D. Same gladius, armor, and shield, but the pilum, javelins, and darts are gone. In their place are two pistols, which are used just as the pila were, just before closing with the enemy. At point blank range the pistol ball goes right through the enemy's shield and armor; just point and shoot and down he goes. Drop the pistol, draw the gladius, and advance to the next enemy, who is likely to turn and run rather than face the same fate. Roma victor!
Of course Rome's enemies are not stupid; they will catch on and adopt these weapons themselves, and adopt tactics to counter them as well. And the Romans will in turn adopt new tactics, especially when facing enemies such as the Persians, with their reliance on cavalry. The development of weapons and tactics will be much like OTL; same game, just different players at a different time and place.