The same statement I made about the Industrial Revolution thread applies here. Getting gunpowder in Classical Greece would be almost ASB due to a myriad of factors. Even if it was discovered, it would see little widespread use and be mostly a niche weapon with limited uses, that would be lost and have to be rediscovered when there are the right economic, political, social, and technological factors enabling widespread use of gunpowder. I guess it could work for a few limited sieges, but be completely useless on the field, and there are several counters to it.
If you apply Carl Sagan's dedication to ancient Greek culture, you can start an ATL that butterflies away the dark ages and allows Mediterranean culture to flourish. We must, though, remember the time lag between Marco Polo's return and the first actual muskets.
But Carl Sagan is a scientist, not a historian, and his ideas rely on several false assumptions and widely discredited ideas, that technology works like it does in Civ games, being a clearly defined tree every people must follow, and not developing in response to various factors, and according to the civilizations' environment and prevailing conditions of the civilization. The Mesoamerican and Andean civilizations alone prove this model wrong, Tenochtitlan was able to sustain a population of 300,000 in the middle of a lake with no draft animals, the wheel, or metal tools. The Inca ruled over one of the highest mountain ranges on earth without horses, writing, or a market economy, and that's just two examples that blow this theory of technological advancement out of the window. That the "Dark Ages" was anything other than a mere paucity of sources which has now been partially rectified, and not this period of ignorance, barbarity, and superstition that no serious scholar today regards as true, but is sadly still prevalent in the popular imagination. That there's this dichtonomy between rational, civilized, and scientific Greeks, and superstitious, ignorant, and barbarous Christians/Muslims who ended Greek and Roman science, which is just wrong on so many levels, usually spouted for modern-day idiotic political reasons.