An announcement of the study, expected Tuesday before the Senate Armed Services Committee, marks a major step toward President Obama's goal of eliminating the military's "don't ask, don't tell" policy, which is based on a 1993 law.
CBS News' chief political consultant, The Atlantic's Marc Ambinder, reports that two sources confirm part of Tuesday's announcement will be an official end to the military's policy of "third-party outing".
According to a source in the Obama administration, Gates and Mullen will announce that third party allegations about a service member's sexuality will no longer be sufficient pretext for an investigation.
Obama has called on Congress to repeal the broader "don't ask, don't tell" policy, but Democrats say they want more guidance on how to allow openly gay service members to serve without causing a major upheaval.
The yearlong study is likely to pave the way for the biggest social change to the military since the 1948 executive order for the racial integration of units.
While his promise is being hailed as a good start by gay rights' activists, Obama is finding resistance in several corners. Some high-ranking military officers are reluctant to embrace the change while troops are stretched thin at a time of two wars.
For their part, Democrats in Congress are unlikely to press the divisive issue until after this fall's midterm elections.
This will probably satisfy Gates, who has long suggested that change should not come too quickly. In a speech last year at the Army War College in Carlisle, Pennsylvania, he noted that the executive order for racial integration took five years to implement.
"I'm not saying that's a model for this, but I'm saying that I believe this is something that needs to be done very, very carefully," he said.
According to U.S. officials, the senior-level study will be co-chaired by a top-ranked civilian and a senior uniformed officer. It would recommend the best way to go about lifting the ban, starting from the premise that it will take time to accomplish that goal but that it can be done without harming the capabilities or cohesion of the military force, officials said.
The officials spoke on condition of anonymity to describe the emerging Pentagon plan ahead of Gates' announcement during his Senate appearance alongside Adm. Mike Mullen, the chairman of the Joint Chiefs of Staff.
One U.S. official told the Associated Press that Gates and Mullen will outline a more lenient standard for enforcing the current ban, as Gates had said last year he would consider. In addition to the change in the third-party outing policy confirmed to Ambinder, the interim measures would also raise the standard for evidence that the service member is gay before the person could be dismissed.
Under the 1993 law, engaging in homosexual conduct - even you don't tell anyone - can been enough to qualify a person for dismissal. The law was intended as a compromise between President Bill Clinton, who wanted to lift the military's ban on gays entirely, and a reluctant Congress and military that said doing so would threaten order.
David Hall, a former Air Force sergeant, said he was discharged in 2002 after someone else reported that he was gay.
"That ended it," said Hall, who now works for a gay rights advocacy group. "Just like that, based off what one person said, ended my dream of getting to fly planes."
In addition to addressing the military's policy on gays, Gates and Mullen planned to outline the military's $768 billion budget for 2011 and another $33 billion requested in war spending this year.
Both Gates and Mullen were expected to underscore the importance of succeeding in Afghanistan, where Obama has ordered 30,000 more troops.
"Our future security is greatly imperiled if we do not win the wars we are in," Mullen said in prepared remarks.
Further, he added, "the outcome of today's conflicts will shape the global security environment for decades to come."
According to figures released by the Pentagon on Monday, 428 service members in 2009 were dismissed for being openly gay compared to 619 in 2008.
Overall, more than 10,900 troops have been dismissed under the policy since 1997.