The protest appears to have started with a story on a fake antifascist Facebook account that was actually set up to troll leftists.
The first reported article about a July 1 Gettysburg antifascist protest was from a Facebook page with only a few hundred followers called Harrisburg100. It published its story on June 14.
The article appears to have been based on an Eventbu page, which has since been taken down. The author writes, "a local group of self-proclaimed anti-fascism activists called ‘ANTIFA’ are planning on holding a rally at Gettysburg National Battlefield on July 1st in protest of President Trump and asks it’s members to 'Bring and Burn Confederate Flags'."
According to Harrisburg100's website, "the mission of Harrisburg100 is to spread civic engagement, and to educate the public on local politics in interesting ways."
The Harrisburg100 article also links to a Facebook page called Harrisburg Antifa, which has about 100 followers. In May, they warned that they were going to be at Gettysburg National Military Park on July 1.
The page's about section reads, "ALERT! ALERT! ANTIFASCIST IN HBG. Fighting Racism, Transphobia, Homophobia and Police Brutality. #Resist #BashTheFash."
Harrisburg Antifa appears to be a troll account and hasn't updated in over a month. The page lists its website as itsgoingdown.org — a general website for antifascists. Also, the page has been called fake by the Central PA Antifa Facebook page, which has considerably more followers.
Trump supporters and far-right trolls have a history of creating fake antifascist social media accounts and then using those accounts to troll real antifascist organizations. A similar thing happened in Boston in March.
A few days later, a YouTuber named TheDelawarePatriot picked up the hoax, urging his followers to "make a stand against Antifa" on July 1.