Under the law, young adults can remain on their parents' insurance plans up to age 26, which will increase the state's costs by $17.7 million in fiscal 2011 alone, according to data provided by the governor's staff.
"We know the real cost though still lies ahead," Deal said during a news conference Wednesday at the state Capitol.
Democrats also gathered to celebrate benefits they said the overhaul produced.
Deal focused on costs to the state. The expansion of Medicaid in 2014 to cover Americans with incomes up to 133 percent of the federal poverty level will add 650,000 new Georgians to the state's Medicaid rolls -- a 45 percent increase, he said. Deal's office estimates that will cost the state $2.5 billion over the next decade.
"It is not going to be sustainable," he said, "certainly not at the state level."
Deal and other GOP governors have asked the Obama administration for more flexibility in managing Medicaid.
"Let us decide how we can tailor a delivery system that we think is more appropriate and would be more cost effective and actually produce better results," Deal said.
President Obama has said there could be some flexibility but with conditions.
Deal withdrew legislation last week that would have created a commission to recommend options for Georgia to create its own health insurance exchange. He said Wednesday that he will appoint a study group to look at the issue.
It would be better for the state to create an exchange than face a federally imposed situation, Deal said. Georgia and other states are still seeking the law's overturn in federal court.
Congressional Republicans will also continue to work toward defunding sections of the law and ultimately repealing it, said U.S. Rep. Tom Price, a Roswell Republican.
In Decatur, volunteers for the Georgia chapter of the Democratic National Committee's Organizing for America project also marked the anniversary.
The overhaul has had a positive impact in Georgia, said Lee Goodall, the director of OFA-Georgia. In addition to young adults being able to stay on their parents' policies, small businesses can benefit from tax credits, he said. About 387,000 Georgians who buy insurance in the individual market will also be protected from insurers who want to drop people who get sick, Goodall said.
"I think people will learn that there are great benefits," he said.