Context: Multivocal Literature Reviews (MLR) search for evidence in both Traditional Literature (TL) and Grey Literature (GL). Despite the growing interest in MLR-based studies, the literature assessing how GL has contributed to MLR studies is still scarce.

Objective: This research aims to assess how the use of GL contributed to MLR studies. By contributing, we mean, understanding to what extent GL is providing evidence that is indeed used by an MLR to answer its research question.

Method: We start by conducting a tertiary study to identify MLR studies published between 2017 and 2019, selecting nine of them. We then identified the GL used in these studies and assessed to what extent the GLs are providing evidence that help these studies to answer their research questions.

Results: Our analysis identified that 1) GL provided evidence not found in TL, 2) most of the GL sources were used to provide recommendations to solve problems, explain a topic, and classify the findings, and 3) 19 different GL types were used in the studies; these GLs were mainly produced by SE practitioners (including blog posts, slides presentations, or project descriptions).

Conclusions: We evidence how GL contributed to MLR studies. We observed that if these GLs were not included in the MLR, several findings would have been omitted or weakened. We also described the challenges involved when conducting this investigation, along with potential ways to deal with them, which may help future SE researchers.