I think the word "gripe" was meant to imply criticism that isn't constructive and has no actual value in improving the end product.

Let's say you send code out for review that is just about perfect how it is. The reviewer is faced with a choice. If they don't suggest any changes, they may feel it gives the impression that you are just good a coder as they are, that their input isn't needed, or that they didn't bother to read it and just rubber stamped it. All of these challenge their place in the organization as a technical thought leader and influencer.

Some people have the maturity and awareness to just say, "Great job. I see no reason to change any of this. Approved." Some people don't, so they will dig for something negative to say. Even if it is just "these variable names are excessively long, and it's distracting to read" or "you don't have enough parameters to warrant the use of the Builder pattern here, so take it out".