A gaffe would be Biden misspeaking in a way that generated negative news coverage.

I am going to use the definition of news conference from UCSB's American Presidency Project: "The term "news conference" refers simply to an interaction between the President and multiple members of the press in a relatively formal setting.  In a "news conference," the President and the press meet specifically for the President to respond to press questions." The APP records any presidential press/news conferences ~monthly I think so if something is ambiguous, I will wait until they make a determination to resolve the market. (Note they also keep track of "Exchanges with Reporters" which I will not count as news/press conferences).

Here is the link to the UCSB's American presidency project https://www.presidency.ucsb.edu/statistics/data/presidential-news-conferences