Also this, which wasn't directly the CIA but by CIA backed groups: https://en.wikipedia.org/wiki/Orlando_Letelier, a political assassination on US soil. (Although not a US citizen)

That's one heck of a leap to blame on the CIA.

To quote the wikipedia article:

A US State Department document made available by the National Security Archive on 10 April 2010 reveals that a démarche protesting Pinochet's Operation Condor assassination program was proposed and sent on 23 August 1976 to US diplomatic missions in Uruguay, Argentina, and Chile to be delivered to their host governments but later rescinded on 16 September 1976 by Henry Kissinger, following concerns raised by US ambassadors assigned there of both personal safety and a likely diplomatic contretemps. Five days later, the Letelier assassination took place.[8]

Documents released in 2015 revealed a CIA report dated 28 April 1978, which showed that the agency by then had knowledge that Pinochet ordered the murders.[9] The report stated, "Contreras told a confidante he authorized the assassination of Letelier on orders from Pinochet."[9] A State Department document also referred to eight separate CIA reports from around the same date, each sourced to "extremely sensitive informants" who provided evidence of Pinochet's direct involvement in ordering the assassination and in directing the subsequent cover-up.[9]

During the tenure of Richard Downie at the William J. Perry Center for Hemispheric Defense Studies, a U.S. Southern Command educational institution located at the National Defense University, the alleged (and as yet unproven) role of Jaime Garcia Covarrubias, a Chilean professor who was head of counterintelligence for DINA in the 1970s, in the torture and murder of seven detainees was revealed inside the center. His alleged role was first brought to Downie's attention in early 2008 by Center Assistant Professor Martin Edwin Andersen, a senior staff member who earlier, as a senior advisor for policy planning at the Criminal Division of the U.S. Department of Justice, was the first national security whistleblower to receive the U.S. Office of Special Counsel's "Public Servant Award."[10] In an October 1987 investigative report in The Nation, Andersen broke the story of how, in a June 1976 meeting in the Hotel Carrera in Santiago, Kissinger gave the bloody military junta in neighboring Argentina the "green light" for their own dirty "war."[11]

I may not be reading this clearly, but how is the CIA involved in the assassination here? It seems like they had intel that it could happen but didn't order it, execute it or have any direct involvement.

The assertion was "the US military can and will assassinate US citizens engaging in constitutionally protected activity in the middle of Manhattan."

It really diminishes your point when that is compared to an airstrike on foreign soil.

I don't really think the territorial claim on the land where the extrajudicial assassination happens is very relevant to the legal fact of the matter.

We don't say that the FSB attempting to execute Skripal in a UK shopping mall doesn't count because it was in the UK. Murder is murder.

The claim that the IC would assassinate Gellman in New York was made by someone who used to be an actual CIA operative and went through their training.

It absolutely doesn't matter where it is. All that means is that the means of murder will change. Instead of being an airstrikes it might be someone off the street paid for the hit or an operative killing them or poisoning them. In both cases, you'd have no way to know. The only places you'd be able to know are in foreign countries where risks of capture and death are much higher and where investigations can't be silenced.