Federal property is public land and the right to assemble there is protected by the First Amendment.
I shall post the First Amendment to make a point:
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.
It says nothing about parks.
The right is not absolute, however. State, local and federal governments can regulate how people express themselves, provided that they adhere to certain principles laid down by the United States Supreme Court.
Notice it says by the Supreme Court. If the court decides something, they can later undecide if they want. The right is not absolute. Schools are public places too, but protests aren't allowed there, for obvious reasons.