I have seen these two online spelling checkers:
https://melobytes.gr/app/orthografos
http://www.neurolingo.gr/el/online_tools/speller
Please note that automatic spell checkers are not 100% reliable. There are words that can change spelling depending on the meaning and context.
For example the words γράφετε (2nd person, plural) and γράφεται (3rd person, singular):
Εσείς γράφετε με μολύβι.
Η λέξη θάλασσα γράφεται με δύο "σ".
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Vasiliki Baskou, Instructor/Director, https://learn-greek-online.com.