The simple answer is: by using a gravity modelling software. There are several software packages on the market, and there are some that are free (open source). Try accessing  this wiki page https://en.wikipedia.org/wiki/Comparison_of_free_geophysics_software

This page contains information on many free / open source software pakages, including some on gravity data modelling. I saw one that combines gravity and magnetic modelling.

THANK YOU MR ROBERTO BUT AFTER HAVING SOFT WHAT IS THE MANER TO CREATE THE SYNTHETIC MODEL IS IT FROM MY DATA OR FROM DOWNLOADABLE FILES FOR GEOMETRIC BODYS REELY  CONFUSED

A synthetic gravity model by validating regional gravimetric geoid determination techniques and computer software.   All topographic masses are modelled

with a constant mass-density of 2,670 kg/m3. Based on these input data, gravity values on the synthetic topography (on a grid and at arbitrarily distributed discrete points) and consistent geoidal heights at regular 1-arc-min geographical

grid nodes may be  computed. The precision of the synthetic gravity and geoid data is estimated to be better than 30 µ Gal and 3 mm, respectively, which  reduces to 1 µ Gal and 1 mm after a second iteration. Gravity and GPS-levelling data gives a realistic representation of the Earth’s gravity field. 

Not sure of your application but if you just want values from a gravity model for comparison, Interpolated values from EGM 2008 are good for comparison with terrestrial and airborne gravity measurements. The 2.5 minute grids and a program to read them can be found at http://earth-info.nga.mil/GandG/wgs84/gravitymod/egm2008/anomalies_dov.html

I just read the grids into MatLab and do the interpolation and any necessary height correction there.

If you provide an idea of what you are trying to use this for I might be able to provide a more useful answer.