Rosas said the decision to step down just one day before the start of the Mavericks' 2013-14 season was "made solely by me."
"I made the personal decision to resign as general manager of the Dallas Mavericks after determining that the position was not the best fit for me at this point in my career," he said.
The surprise move came after Mavericks owner Mark Cuban in August recruited him from the Houston Rockets, where he had worked in the front office for 10 years.
Rosas served as the Rockets' executive vice president of basketball operations last season.
With the Mavericks, his GM job was considered junior to that of team President Donnie Nelson, the Dallas Morning News reported.
"I was disappointed when Gersson told me of his decision, but I understand and all of us wish him the best," Cuban said.