Mark’s references to movements across the Sea of Galilee are impossible to trace sequentially. Mention of specific location near the sea are either unknown sites, such as Dalmanutha (8:10), or are patently inaccurate, as in the designation of the eastern shore of the lake as the country of the Gerasenes (5:1)" (Kee, loc cit). Gerasa is more than thirty miles southeast of the lake, too far away for the setting of the story which demands a city in its vicinity, with a precipitous slope down to the water. This and other examples have lead many to believe that the authors were not personally familiar with the geography of the land.
The fact that the Gospels were written in Greek does not provide conclusive proof in and of itself. It is simply a probability statement which indicates how likely it would have been for a Aramaic speaking Jew from the first century Palestine to write his gospel in a foreign language while quoting a foreign translation of his own Hebrew Bible. It is not impossible but just not probable. It needs to be weighed against the rest of the evidence.