In Greece and Cyprus, most opinion polls indicate opposition to gay marriage, however, the most recent one puts 'for' slightly ahead. Probably just an outlier though. Most Greeks I know would vote against it and those who might be for it don't care. Thus, I think it would be low turnout and a lopsided win for against. If turnout was higher, the margins would be closer but I think against would still win. However, for or against gay marriage has never really been a big issue in Greece because the focus is on other things, namely the economy and perhaps with a campaign on the issue, for could win. That being said, I put it at 80% chance of against victory.
Some polls:
http://assets.pewresearch.org/wp-content/uploads/sites/11/2017/05/09154356/Central-and-Eastern-Europe-Topline_FINAL-FOR-PUBLICATION.pdfhttp://www.equineteurope.org/IMG/pdf/ebs_437_en.pdfhttps://www.dianeosis.org/wp-content/uploads/2017/03/ti_pistevoun_oi_ellines_2017_.pdf
