On the I-485, which you file to get a GC, you have to answer several questions like this:
> Do you intend to engage in any activity that could endanger the welfare, safety, or security of the United States?
> NOTE: If you answered "Yes" to any part of Item Numbers 42.a. - 45., explain what you did, including the dates and location of the circumstances, or what you intend to do in the space provided in Part 14. Additional Information
> Recruited members or asked for money or things of value for a group or organization that did any of the activities described in Item Numbers 43.b. - 43.e.
If you say 'yes' to these, you probably aren't getting a GC. If you falsely say 'no' to these, you may have committed fraud. The reference to Item Numbers 43.b - 43.e can be found by reading the I-485 - https://www.uscis.gov/sites/default/files/document/forms/i-4... - but to save you time, it seems to apply to any group doing armed resistance.
That said, mere speech supporting Palestine is, as you say, legal. I also think that it had to be false at the time the statement was made, not something he only did afterwards. But if they can show that a person lied on these questions or any of the other several dozen questions in the application, they can accuse them of obtaining the GC fraudulently and go into removal proceedings.
Reading between the lines, this is what I believe is happening to Khalil based on statements given in articles like - https://reason.com/2025/03/13/mahmoud-khalil-is-an-easy-call... - compare the questions I quoted to their stated justifications in that article:
> The official said that Khalil is a "threat to the foreign policy and national security interests of the United States."
> "The allegation here is not that he was breaking the law," said the official. "He was mobilizing support for Hamas and spreading antisemitism in a way that is contrary to the foreign policy of the U.S."
Now, I'm not exactly sure exactly how removal proceedings work, but from what I've read, it seems likely that he'll get some kind of hearing. Hopefully, this gets adjudicated properly, promptly and fairly in a way that respects his first amendment rights, though it is concerning that someone can just be held in detainment waiting for all this.