What canon media does the Ghorman massacre come from?
Started with an RPG sourcebook back in the 90s - The Rebel Alliance Sourcebook, 1990. In that Legends version, the Ghorman massacre involved Tarkin (and how Tarkin was promoted to a Moff) and was the event that pushed a lot of Senators (like Bail Organa) to join up with Mon Mothma in starting the rebellion.
In the newer Disney canon, it pops up in the
Forged in Battle sourcebook for Fantasy Flight Games' Star Wars: Age of Rebellion roleplaying game (2016). Then, it got name dropped in the Star Wars Rebels episode "Secret Cargo", which is totally worth rewatching that Rebels arc as it is heavy in Mon Mothma/early Rebellion stuff probably very closely related to what's going to happen in Andor's later arcs.
It pops up in a lot more detail in the "fake historical non-fiction" book Star Wars: The Rise and Fall of the Galactic Empire (which is an awesome book - it's supposed to be an in-universe history textbook that looks back on the empire like our history books look back on WW2 or something). Also pops up in various places over the years (Sansweet's Star Wars Encyclopedia from late 90s; Rogue One Visual Dictionary, etc.).
One of things that's been referenced and referred to by lots of things but never dealt with this directly until Andor Season 2.