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It's believed that Mars lost the majority of its atmosphere not because of a lower gravity compared to Earth, but because Mars lacks a magnetic field.
The Sun emits a continuous stream of charged particles called the solar wind.
Earths molten iron core produces a magnetic field that mostly deflects the solar wind, but some leak through at the Earths poles cause the aurora. Without this magnetic field the solar wind would slowly strip the atmosphere from a planet.
Mars' iron core cooled fast, likely because of its small size, and its magnetic field shut down around 3 billion years ago. Before the core cooled it's believed that Mars had an atmospheric pressure similar to Earth. The NASA MAVEN probe determined that Mars is currently losing atmosphere at at rate of 6kg per minute.
Interestingly, the solution to the Mars atmosphere problem may come from Venus. Venus has an atmosphere but no core generated magnetic field. ESA Venus Express indicated that a magnetic field was being generated by an interaction between the solar wind and the Venus' ionosphere. Maybe we could make this happen on Mars.
Chris
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