There's an overwhelming amount of evidence that gratitude is inversely correlated with depression, and a good amount that shows it to be useful as an intervention as well. Even the article that you provided as evidence to the contrary found positive effects, just not dramatically higher than placebo.

For example, here are the first couple pages of results I got for gratitude studies. Every single one found a positive affect.

https://journals.sagepub.com/doi/abs/10.1177/135910531557245...

https://www.tandfonline.com/doi/abs/10.1080/0144341090349393...

https://link.springer.com/article/10.1007/s10902-014-9542-3

https://www.cambridge.org/core/journals/spanish-journal-of-p...

https://www.sciencedirect.com/science/article/abs/pii/S00926...

https://synapse.koreamed.org/search.php?where=aview&id=10.12...

https://www.sciencedirect.com/science/article/abs/pii/S01918...

https://www.tandfonline.com/doi/abs/10.1080/15298868.2015.10...

https://psycnet.apa.org/record/2016-51687-001

https://onlinelibrary.wiley.com/doi/abs/10.1111/sjop.12252