Where are the Seleucid Greeks?
(InB4 Kalash people. We’re pretty sure they’re not Greeks.)
One can only presume, they assimilated. The ruling class would have been Greek for a fair while; royalty certainly was. But there’s no reason to think the majority of Greeks didn’t intermarry. Not that we’d know much about it, because the contemporaries didn’t pay that much attention to ethnic difference. Lucian of Samosata is quite happy to tell us he’s “Assyrian”, and wrote a book on the religious history of Syria; but everything he wrote was firmly enmeshed in Greek literary culture.
What did they leave behind? The architecture of places like Palmyra. The tradition of human-looking statues in India, including statues of the Buddha. A Greco-Bactrian letter in Unicode: Sho (letter). Awe of Greek learning, inherited by the Arabs. Lucian himself. The koine of the New Testament (although the vehicle of Eastern Orthodoxy was Syriac, not Greek). And lots of stories about Alexander, including those involving Dhul-Qarnayn.