Can you confirm a few words for me. By gradiometer, do you mean a magnetic gradiometer? Gradiometer just means a gradient or difference over a fixed distance, it doesn't imply the method used for the measurement. There are a few kits that I have seen to build fluxgate magnetometer gradiometers. I think if you search the forums here you may find them.
I personally use QuSpin Total Field Magnetometers Generation 2 (QTFM-Gen2). They are solid state atomic magnetometers that are really small and very power efficient. I use them in man-carried configurations, bicycle carried configurations, as a towed array behind my Kawasaki UTV, and as drone mounted magnetometers. They are about $10K each and I have 11 spread through the different deployments.
All of the software from Golden Software, such as Grapher, Surfer, or Voxler are just visualization programs. They aren't data processing or geophysical inversion programs. Are you asking how to process magnetic gradiometer data? The first few steps are to clean up the data quality, filter the data, and fix and positioning issues. I'm sure there are a few free programs, but I use PetRos EiKon QCTool as it is pretty cheap. The basic software is less than $400 and the magnetic data processing add-on is $800, which is as close to free as you can get for geophysics software.
