Huh? I'm not sure why someone at T-Mobile would have told you that. Maybe they aren't familiar with the Pixel. That phone is quad-band GSM with UMTS bands 1 & 8 plus a boat load of LTE bands including 1, 3, 7 & 8. I'd say you're golden. My wife's Nexus 5X was the better of our two phones in Ireland, which uses the same bands and technology as Greece and Italy.
P.S. I think I know how you got that answer from T-Mobile. The Pixels are not T-Mobile-supported phones. They aren't on T-Mobile's list. When you go tot he international roaming page, enter Greece and then put "Pixel" in the box that says, "See if your device works in Greece." it comes up blank.
It will work in Greece, the only reason it doesn't show here:
Is because doesn't sell it so they can't guarantee it.
Check pages like these:
Get Greece's available Bands/Freq then compare it to your phone