Hellenist is a word that means one who speaks Greek, not simply a Greek person. In the New Testament it is only used in Acts, and referred to Greek-speaking Jewish Christians (Acts 6:1 ESV), Greek speaking Jewish non-Christians (Acts 9:29 ESV) and more. The country of Greece itself is written Hellas in Greek so the reference simply to “Hellenists” in Acts 11:20 could refer to Greek speaking gentiles (not Jewish or Christian), which is why some translations actually just say “Greeks” (for example the NIV). But in it’s basic form Hellenist could refer to anyone from any country who assimilated into Greek culture. In any case, through the book of Acts it’s clear the gospel was reaching 1) the Hebrew Jews, 2) the Greek-speaking Jews and 3) Greek-speaking gentiles (everyone in other words)!
Here is David G. Peterson in the Pillar New Testament Commentary on Acts 11:20:
“Part of the confusion is Luke’s use of the term ‘Hellenists’ in 6:1 to describe Greek-speaking Jewish Christians, but in 9:29 with reference to Greek-speaking Jews who were not Christians. I agree with Witherington that the context defines more precisely in each case what sort of Greek-speaking person Luke has in mind, ‘ranging from Jewish Christians, to Jews, to pagans’. ‘Greeks’ would be more naturally natives of Greece, but ‘Hellenists’ in this context would be Greek-speaking Gentiles more generally.” (David G. Peterson, The Acts of the Apostles, The Pillar New Testament Commentary (Grand Rapids, MI; Nottingham, England: William B. Eerdmans Publishing Company, 2009), 353.)
Adorn the Gospel 