Who is the only Greek to win a modern Olympic gold medal in an athletics event?

In 1896 Summer Olympics were held in Athens, Greece. Spyridon Louis won the first modern-day Men's Olympic Marathon for the host nation, and given the race's inception, that was rather fitting.

Just five nations have attended every Summer Olympic Games, Greece being one of them.

https://en.wikipedia.org/wiki/1896_Summer_Olympics
https://en.wikipedia.org/wiki/Greece_at_the_Olympics

Although her victory is highly controversial due to failed drug testing, Fani Chalkia won the gold medal in the 2008 women's 400 meter hurdles.

https://en.wikipedia.org/wiki/Fani_Chalkia

Let's not forget Miltiades Tentoglu who won the long jump at the 2020 Tokyo Olympics, held in 2021.
https://en.wikipedia.org/wiki/Miltiadis_Tentoglou
In fact, a total of 8 Greeks have won golds in athletics at the Olympics.
Here are the other seven:
Spyridon Louis - 1896 - Men's marathon
Konstantinos Tsiklitiras - 1912 - Men's standing long jump
Voula Patoulidou - 1992 - Women's 100 metres hurdles
Konstantinos Kenteris - 2000 - Men's 200 metres
Athanasia Tsoumeleka - 2004 - Women's 20km walk
Fani Chalkia - 2004 - Women's 400m hurdles
Katerina Stefanidi - 2016 - Women's pole vault.

Meanwhile, Miltiades Tentoglu has won his second gold medal at Paris 2024. Here's the winning jump:

https://www.youtube.com/watch?v=bWNKVCpsO_s