Short answer: Yeah, I guess...
- There is a section on Wikipedia discussing a host of issues with measuring the effectiveness of psychotherapy. I won't review that here.
- Gestalt therapy is not a particularly popular form of psychotherapy. For example, an APA survey by Norcross & Rogan (2012) reported 4% of respondents ascribing to Gestalt, and in a global survey by Goodyear et al (2016) only about 2% of respondents reported Gestalt as their preferred theoretical orientation. Even fewer therapists practice Gestalt exclusively.
- Consequently, there is a paucity of research data available on the effectiveness of Gestalt therapy. For example, Raffagnino (2019) found only 11 studies in 12 years for their review of the effectiveness of Gestalt therapy, most of which are small, and do not report comparable effect sizes.
For the Love of Dodo Birds:
But it turns out that thanks to a character in Alice in Wonderland, we may reasonably answer the question without much additional evidence. This is because most "empirically validated" treatment modalities have been found to be equally effective - a finding known as the "Dodo bird conjecture" - with a "moderate" level of effectiveness all around. The so-called "third wave" therapies - those built on CBT, with outcome measures in mind - may be an exception to this rule, but Gestalt therapy is an older orientation, so would reasonably fall under the umbrella of moderately effective treatments. Studies measuring effect sizes for Gestalt therapy support this conclusion (eg, Stevens et al, 2014; Elliot et al, 2013; Shinohara et al, 2013; see Brownwell, 2016 for more examples).
A summary of a notable meta-analysis by Uwe Strümpfel (2006) of approximately 4500 patients concludes (original in German):
The studies confirm the effects of Gestalt therapy in a wide range of
clinical disorders such as schizophrenia, other psychiatric and
personality disorders, affective disorders and anxiety, substance
dependencies and psychosomatic disorders and for work with special
groups and in psychosocial preventive health settings.
The results published in this book are evidence that Gestalt therapy
does not lag behind other forms of therapy in regard to efficacy and
breadth of applications and even demonstrates particularly good
treatment outcomes in certain areas of change.
If you are interested in the scientific basis of psychotherapies in general, then check out:
As is often the case in medicine, tests of effectiveness are agnostic about the scientific basis of treatment. It does not matter if therapy involves extremely well-researched conditioning paradigms, or more questionable practices of flashing lights into people's eyes. There are for example, many art-oriented therapies, and no one would argue that art is science-based - what matters is the effect, even if the successful treatment is literal snake oil. Gestalt therapy being largely client-centered (rather than scientific) is not much of a concern in this sense.