The vote abstention should be right, I searched on the greek voting system and I don't see any requirement besides[1]:

* having the greek citizenship

* being older than 17

* Not being dissalowed by the court to vote

All citizens get automatically added to the "registered vote" after they turn 17.

E.g. A greek citizen is always a registered voter even if he doesn't go and vote.

If we take the 2019 numbers we have:

9961718 registered voters

4192215 voters who didn't fill out the forms (I assume non voters are counted this way)

(4192215×100)÷9961718 ~=42% Are non voters

[1]https://en.wikipedia.org/wiki/Elections_in_Greece

[2]https://de.wikipedia.org/wiki/Parlamentswahl_in_Griechenland...

Edit: formatting, add a note about what the 42% means