The address to rebels is Gadahn's second appearance in September in al-Qaida videos posted on jihadist online forums, CNN reported.
The video, "Advice and Support to Our Rebel Brothers Against Injustice," was released last week and reportedly was produced in April by al-Qaida's media unit.
The two videos are the first Gadahn videos since before Osama bin Laden was killed during a U.S. raid on his compound in Abbottabad, Pakistan, last year.
Gadahn's remarks also were included in Ayman al-Zawahiri's message recorded for the anniversary of the Sept. 11, 2001, terrorist attacks on the United States.
Since he first appeared in an al-Qaida propaganda video under the name Azzam al-Amriki in 2004, Gadahn, of California, has been in numerous videos disparaging the West because of his appeal to Western Muslims and his knowledge of Western culture, observers note.
Unlike American Anwar al-Awlaki, who played a significant operational role for the Yemeni al-Qaida offshoot before his death in a drone strike last year, Gadahn appears to be expanding his role in communications for al-Qaida, CNN said.
"Gadahn appears to have carefully crafted a role for himself in the organization and through competent performance, has presumably garnered the respect necessary to expand that role," analysis by the Combating Terrorism Center at West Point, N.Y., indicated.