It was an impassioned plea for him to take action to stem the number of military suicides, which last year exceeded the number of those who died in combat in Iraq and Afghanistan, according to a Congressional Quarterly report.
On Wednesday, after taking a week to consider her position and learn about efforts the military has undertaken to guard the mental health of troops, she penned another letter.
''What can I do to help you?'' she asked the president. ''How can we get to the bottom of why this is happening?''
Gates knows that her son, Airman 1st Class Austin H. Gates Benson, 19, of Hellertown, believed wholeheartedly in the good the U.S. military is doing in Afghanistan, but the horrors of war became too much for him. He shot himself May 3.
He was stationed at a remote combat outpost in Khyber, near the Pakistani border, where he was helping to establish communications. It was a region that had recently suffered civilian deaths in a Pakistani airstrike.
Gates Benson was known by his friends and family as a man who fought for his principles. He enlisted in the Air Force shortly after graduating from Saucon Valley High School in 2008.
The voice of the student body at Saucon Valley, Gates Benson believed forcing students to wear ID badges infringed on their rights. He often debated the issue and other topics with school administrators.
Friends say he got his stand-up character from his mother.
''I tried to raise him to be responsible, to think things through, to look beyond the first thing he saw,'' she said. ''Today I can't help but think I got exactly what I wanted.''
Gates believes her son sacrificed himself for peace, ''so he'd never be responsible for the loss of another life.''
She has been moved by the outpouring of love and support from his fellow airmen, his ''family'' at Robins Air Force Base in Georgia, where he was assigned to the 54th Combat Communications Squadron.
''I do not grieve alone,'' she said. ''These people grieve along with me. I had no idea how hard this was for them.''
A surge in suicides has been a troubling fact in all branches of the military. Last year Air Force officials reported 41 active-duty suicides, while 52 Marines and 48 sailors took their own lives. The Army reported the most suicides, nearly 170.
Moved by the numbers last year, the Army created a suicide prevention task force to examine all of the recent suicides and try to find commonalities.
There is no easy solution, Gates said. She believes the problem is more complex than she can begin to imagine.
''I believe that the military has done everything that it can,'' she said. ''They are at a loss.''
As she sat in her living room in Hellertown on Wednesday, surrounded by pictures and other keepsakes she has gathered for the memorial service on Friday, Gates said her son was a rare gem, a young man who felt keenly the plight of others.
In Afghanistan, surrounded by abject poverty, he told his mother he'd always be appreciative of his life in the U.S. He believed so strongly in the mission in Afghanistan that he had requested to extend his tour, Gates said.
He always wanted to be in the military, she said. As a boy, he played with G.I. Joe action figures. In high school, he joined the Civil Air Patrol in Quakertown and had decided to enlist in the Air Force before he graduated.
Before he went to Afghanistan, he and his friends got tattoos. On his forearm, Gates Benson had inscribed the Latin greeting ''Si vales, valeo,'' which means ''if you are well, then I am well.''
For his high school yearbook he chose to quote Oscar Wilde: ''Anybody can make history. Only a great man can write it.'' But his other favorite saying was from Edmund Burke: ''All that is required for evil to triumph is for good men to do nothing.''
For Gates, those three sayings sum up what her son stood for.
''He was selfless and he died for what he believed in,'' she said. ''I guess that would be his epitaph.'