Hi Raoul

Mars has plenty of gravity to retain atmosphere under present conditions. Two processes eroded its early atmosphere, neither of which are really relevant anymore - enhanced early solar wind and impact erosion. No magnetic field and lower escape velocity meant both were quite efficient, eliminating ~ 95-99% of the atmosphere that once might've been. Impactor frequency has declined to insignificance and the solar wind won't get as vigorous again until the Sun is a red giant.

Jeans escape - 'gravity loss' - of atmosphere is quite ineffective on Mars at any time, now or then. Because the atmosphere was, and is, mostly CO2 the exosphere never gets very warm (~200 K) - unlike Earth's N2/O2 atmosphere (exosphere 1000-2000 K.) Thus it is well able to retain CO2 and even hydrogen retention has been measured to be more effective than first believed.