The amendments, crafted by parliamentarians from both ruling and opposition groups, will turn Zardari into a ceremonial head of state.
But analysts say he still will maintain his hold on the Pakistan People's Party (PPP) and keep Gilani's loyalty.
Two of the biggest privileges of past presidents - soon to be lost by Zardari - were the power to dissolve the National Assembly and appoint the heads of the armed forces and judges.
Mohammad Zia-ul-Haq, the former president, introduced these powers in the 1980s to maintain control of the government.
In the 1990s, the National Assembly and four provincial assemblies were dissolved three times, ousting the governments of former prime ministers Benazir Bhutto and Nawaz Sharif.
A leader in the main opposition party, the Pakistan Muslim League-N - the party of Zardari's chief rival, Nawaz Sharif - urged the government to focus on resolving chronic problems, including inflation, power shortages and unemployment.
"The prime minister's responsibilities have increased," Nisar Ali Khan said.
"Now he has no excuse; this government has no excuse but to resolve people's problems.
"Today, we all have got to give a commitment that no politician, no Pakistani will support a military dictator," he said.
The military has ruled Pakistan for about half of its 63 years of independence.