how can I build probability maps with gmapping

edit

It's not gmapping that is using only three discrete values, but the nav_msgs/OccupancyGrid.msg message format. Internally, gmapping uses probabilistic cell values, but in the slam_gmapping.cpp source, starting in line 631, those are converted to above linked message format like so:

for(int y=0; y < smap.getMapSizeY(); y++)
{
  /// @todo Sort out the unknown vs. free vs. obstacle thresholding
  GMapping::IntPoint p(x, y);
  double occ=smap.cell(p);
  assert(occ <= 1.0);
  if(occ < 0)
    map_.map.data[MAP_IDX(map_.map.info.width, x, y)] = -1;
  else if(occ > occ_thresh_)
  {
    //map_.map.data[MAP_IDX(map_.map.info.width, x, y)] = (int)round(occ*100.0);
    map_.map.data[MAP_IDX(map_.map.info.width, x, y)] = 100;
  }
  else
    map_.map.data[MAP_IDX(map_.map.info.width, x, y)] = 0;
}

Haven't played around with the code, but the occ variable should hold the probability of occupancy.

See also this question/answer about visualization in rviz.