There was very ponctual relations, mostyl transiting trough India, and romans merchants often encountered chinese merchants in the indian harbours.
Rome was known as Da Qin "The other China" or "The great China", and China as "the country of the Seres", the makers of silk.
But even if Gan Ying reach Rome, it wouldn't change anything. China and Rome are simply to far each other to make peace, exchanges etc. The technological gain would by likely as OTL, by the Slik Road.
I doubt you'll have anything else than an anecdotical "hey, we encountered an ugly foreigner today at Rome." At best, though unlikely, you'll have sort of reverse Marco Polo that wouldn't gave that much technological advises (after all Marco Polo never exported the idea of european rudder, different but better than the chinese one)