It is amazing how knowing just a few words of Greek, including appropriate common social rituals, will remove one a little from the class of generalized foreigners with whom Greeks cannot exchange the simplest formalities, and who are therefore inaccessible entities to be related to with improvised sign language.
It is a good thing for visitors to learn the difference between the singular/familiar 'Ya sou!' and the plural/formal 'Ya sas!' which means, in both cases, 'Your health!' and which can be said both on arrival and on parting without error (though there are other greetings and words of farewell, too, as seen below).
'Ya sou!' can only be used with one person, since it is singular, and 'Ya sas!' when addressing more than one person, but if a single person being greeted is up in years (or just a good deal older than oneself), and/or a stranger, to be on the safe side, use the plural/formal 'Ya sas!'.
Get a good phrase book with not only the transliterated phrases in your own alphabet, but also in the Greek, and carry it with you. Not only will that give you a handle on the Greek alphabet, but you can always show someone the question you are asking in the book if they don't understand you (since it's all too easy to mispronounce Greek words from transliterations).
And if you intend to be in Greece for up to a month, knowing the Greek alphabet will enable you to read signs (both in small letters and capitals), so that you can locate places more easily are are in general more comfortable. Knowing some basic Greek is a sure ice-breaker when travelling in Greece.
People really appreciate the effort that you've made, since so many make none whatsoever, and even ask Greeks questions in English (or other languages) without asking the Greeks they are addressing if they speak it.
In tourist offices, most employees must and do know English (and sometimes French and German), but many Greeks don't, and it is rather arrogant to assume that they do. Learn at least how to say, 'Excuse me, do you speak English?' (French, German, etc.) 'Signomi, (accent on the 'o'), Milate Anglika?' (Gallika, Yermanika?) (Milate with accent on the 'a', pronounced 'ah', and on the last 'a' in Anglika, both 'a's pronounced 'ah'.