I can understand why GitLab wants to pad out their reviews for this site because to the casual observer it only divulges a few written reviews before clamming up and demanding a visitor divulge salary and other personal information to see any more. So if the top reviews seem glowing you may be getting a false impression.
More generally I wonder if Glassdoor is like Yelp and other social review websites. I'm sure companies affected by negative reviews are contacted by Glassdoor sales reps offering to "improve" their image for money (i.e. make bad reviews disappear). And there will be external services trying to boost or extort companies for $$$ by posting fake reviews that move the rating one way or the other.