Gibson appeared on ABC's "Good Morning America" on Thursday and said he had an "ethical problem" with the proposition, which would make about $300 million available annually for 10 years to fund the controversial research. The measure would allow cloning for medical research only.
Gibson has recorded a 60-second message that he gave to the campaign Thursday. A spokesman for the anti-proposition campaign said officials are still discussing how to use the message.
"It's kind of a last-minute thing but we are glad he could join," said No on 71 spokesman Tim Rosales. "He's a great spokesperson."
Gibson also told "Good Morning America" he had called Gov. Arnold Schwarzenegger - who supports the California ballot question, Proposition 71 - on Wednesday night to talk about the issue - but the governor told him he had to make a speech and would call back.
"Well, Arnold, I'm still waiting for your call," Gibson said.
Later, Schwarzenegger appeared puzzled.
"I don't know what this was all about," the governor said in Los Angeles.
"I did talk to him for several minutes and explained to him what my position was on" the stem-cell proposal, the governor said. Schwarzenegger said he had to cut short the call because he had to give a speech in San Diego.
"After that, I called back at 9 o'clock and left a message on his phone," Schwarzenegger said. "He hasn't returned my call."
Gibson, a Roman Catholic, as is the governor, said when he first heard about the proposition he was overjoyed but changed his mind. Gibson said he had no faith in the cloning of human embryos but would support the use of adult stem cells.
"I found that the cloning of human embryos will be used in the process and that, for me, I have an ethical problem with that," he said. "Why do I, as a taxpayer, have to fund something I believe is unethical?"
Earlier in the week, Brad Pitt voiced support of the measure.
"We have to make sure that we open up these avenues so that our best and our brightest can go find these cures that they believe they will find," the actor said. "Proposition 71 accomplishes this."