Hi Morteza,
In the on the hard drive section in this tutorial : https://neuroimage.usc.edu/brainstorm/Tutorials/HeadModel it is noted that the gain matrix is saved in the following way:
The Gain matrix is the most important piece of information in the structure. It stores the leadfields for 3 orthogonal orientations (x,y,z) at each grid point (p1, p2, etc). The information relative to each pair sensor <-> grid source point is stored as successive columns of the matrix are ordered as: [ p1 _x, p1 _y, p1 _z, p2_x, p2_y, p2_z ...]. For the tutorial introduction dataset, with 15002 sources, the gain matrix has 45006 columns.
With this information, you can find the grid points that you want to use.
Kind regards,
Steven