I just came across this convenient list of primary source quotes about pre-Christian usage of euangelion and euangelia by Glen Davies. I’ve seen the references in the TDNT article but never looked them up. But it is quite interesting to see them set out. For example there is Plutarch’s Moralia 374d which says:
“Why, as we are told, the Spartans merely sent meat from the public commons to the man who brought glad tidings [εὐαγγέλιον] of the victory in Mantineia which Thucydides describes! And indeed the compilers of histories are, as it were, reporters of great exploits who are gifted with the faculty of felicitous speech, and achieve success in their writing through the beauty and force of their narration; and to them those who first encountered and recorded the events [εὐαγγέλιον] are indebted for a pleasing retelling of them.”
Note that euangelion here is associated with news of victory and the recording of an event that brought the victory – sound familiar?