In the Gupta dynasty era, trade was so prolific and governance so beneficent that wealth accumulated and a strong and prosperous middle class developed (in contrast to the Mauryan dynasty) in which cultural pursuits and the arts were enjoyed in the leisure time wealth created.
I went to Google Books to search for material relevant to this topic. I narrowed the terms of the search a bit, but one of the great things about Google Books is that you can search the same basic topic in numerous ways by changing the search terms slightly. Don't forget, too, that many of the books dealing with a topic such as this are likely to contain bibliographies that will point you in the direction of other relevant material. So, within each book that looks relevant, you may also want to search for such terms as "Bibliography" or "Works Cited" or "Annotated Bibliography." In any case, here's what I found on the topic in which you are interested:
I think #4 is right to indicate that good are not the only thing that travel along important trade routes. Clearly the diffusion of ideas and new technology is something that is just as important, if not more so, than the goods themselves and the economic advantages of such trade routes. This was clearly important in the development of these early civilisations.
One major effect of trade ties in the Gupta Empire was the spread of scientific knowledge. Aryabhata, a scientist, was one of the first people to propose a spherical Earth with rotation, reflection of sunlight on the Moon, and eclipses based on casting shadows instead of unseen objects in the sky. His theories spread throughout the trading nations and were valuable in later scientific thought.
Seagoing trade was dominant during the Kushan Empire. As number 2 mentioned, the Silk Road brought trade with Rome, where they traded Chinese silk. The Silk Road was an important and sophisticated trade route.
Trade had a huge importance in the time of the Kushan kingdom. The Silk Road that connected the Roman Empire and China ran through this kingdom's territory. It brought both economic wealth and new ideas and technology to the Kushan kingdom from such places as China.