Establishment of travel agencies by enterprises originating in non-EU member countries is subject to a reciprocity requirement.
"Article 3 - Notification of the operation of a [travel agency] [...]
2 . Natural persons, third-country nationals, as well as legal entities established in third countries have the right to operate a [travel agency] if they make a special power of attorney in Greece. The above right is recognized under the condition that according to the legislation of the state of their citizenship or registered office, the same right is recognized for Greek citizens and for companies based in Greece.
Otherwise, the natural or legal persons mentioned in the first paragraph are entitled to provide tourist services only if they contract for this purpose with a [travel agency] operating legally in Greece.[...]" (Law on the establishment and operation of travel agencies (393/1976), last amended by Law 4811/2021, Article 3 [google translation based on unofficial text at kodiko.gr])
Law on the establishment and operation of travel agencies (393/1976), last amended by Law 4811/2021, Article 3
[<a href="http://www.et.gr/idocs-nph/search/pdfViewerForm.html?args=5C7QrtC22wEE8HdDZpIXTHdtvSoClrL8Rc9TCA8iZVd5MXD0LzQTLWPU9yLzB8V68knBzLCmTXKaO6fpVZ6Lx9hLslJUqeiQfUVZwUjpGqGjuKbnui3IWqquhuamS5KFPPAypihm2hY.">Law (official)</a>],[<a href="https://www.kodiko.gr/nomothesia/document/326631">Law (unofficial)</a>]