Yia seems more acceptable, Gaia is too close to the word used for Earth (Gaea/γη). You still might be able to pass it for a hello, but it's common when using phonetic Greek (using the English alphabet to communicate in Greek) that you would say either "Yia", "Yiasou", or something along the lines of that.
Theseus, there are a few Greeks playing, you can find them much easier now that the flag system was introduced, but here are some that I play with often:
https://www.warlight.net/profile?p=688346430https://www.warlight.net/profile?p=2731867411...and pretty much everyone in this tournament:
https://www.warlight.net/MultiPlayer/Tournament?ID=13371