I have done some gravity surveys. We currently have an old Lacoste & Romberg gravimeter. My wish list currently includes a Scintrex CG-5 gravimeter, but since they cost $80,000 and have a one year waiting list I won't be getting it very soon, which is a relative gravimeter. There were three manufacturers of gravimeters until a few years ago. Scintrex, Micro-G and Lacoste & Romberg all merged into one company.
Building a gravimeter is not a simple DIY project. It might be the most difficult geophysical instrument to design and build. The absolute gravimeters from Micro-G are amazing instruments that cost up to $500,000 and take almost a year to build. The gravimeters a measuring a signal that is nine orders of magnitude smaller that the values we normally use (e.g. 9.8 m/s2).