It seems very likely that merging neutron stars are a major contributor to some of the (heavier) elements in the universe. These elements are made by rapid neutron capture onto pre-existing seed nuclei - known as the r-process. The process could work well in the hot ejecta created by the merger of two neutron stars.
Whether this mechanism dominates the creation of these elements is still uncertain. Firstly, there are other possible r-process sites - most obviously, supernovae, that may also be important. Secondly, in order to compare the production rates in neutron star mergers with the abundances of heavy elements in say the Solar System, we need to know what the rate of these neutron star mergers is, as a function of cosmic time, and we need to know how much of what chemical elements are produced in the merger.
What the new LIGO gravitational wave observations give us, is some very approximate idea of the rate at which these mergers are happening. But it is still order-of-magnitude stuff. The second important thing is that via the identification of an optical/infrared counterpart to one of these mergers one is able to study the chemical composition of the ejecta in some detail. These observations are broadly consistent with the idea that the ejecta contain a lot of material that is produced in the r-process, though don't really give much information on the abundances of individual chemical elements.
I would conclude that the one detection of GW170817 has moved us from an upper limit on the merger rate to an order-of-magnitude estimate, and the identification of the signatures of r-process elements in the ejecta tells us that mergers can produced heavy elements in some quantity. This is consistent with the notion that most of some specific heavy elements are produced in this way, but the detailed quantitative evidence for that is still some way off.