I'd challenge that notion. Rome had several writers who made major contributions to science, notably Vitruvius and Frontinus, who did, admittedly focus more on the application of science than scientific discovery. If you want someone more in line with the Greek natural philosophers, Lucretius is a better example. He wrote about Epicureanism and atomism, and in his De Rerum Natura discussed the origin of matter, the world, and life at length.

If you want to look at lasting scientific achievements that came from the Romans, similar to your example of gunpowder, there are many achievements, such as aqueducts, plumbing, and numerous building techniques.

Galenos, commonly known as Galen to us, was a famous physician who's work and writings advanced medical science. Though himself a Greek, he worked in Rome during the second century CE. Even if he wasn't of Roman extraction, I think it's safe to say he was part of the Roman imperial system - I don't know if you want to count that or not.

Galen could perform cataract removal on humans and made many studies of animal anatomy. His works were hugely influential and continued to be used well over a millennia after his death. It's been a while since my Roman history stuff at university so apologies for the scant info. Hopefully someone with more information can elaborate for you, if you think Galen counts.

There are other architects as well beyond Vitruvius. Check out the works of Rabirius and Apollodorus of Damascus.

Also, Pliny the Elder was science-ish and there were agricultural commentaries such as Varro and Columella.