The GRAT, GQ-6 and GAC is the most common tools used to measure gratitude at the state and disposition level. Check out McCullough's paper (The Grateful Disposition: A
conceptual and Empirical Topography.) at the link below:
They described the development of the gratitude questionnaires along with the psychometric properties of those scales.
In addition to the responses already given, you may be interested in the comparative validation of the Short GRAT and GQ-6 performed by one of our PhD students (see link).
Jans-Beken concludes:
"Comparison of the outcomes of the SGRAT-NL with the outcomes of the GQ6-NL regarding reliability and validity in this study shows that there is great resemblance between both scales. The outcomes indicate that both scales are of sufficient psychometric quality to be used for assessment of the grateful disposition in individuals and groups (Kruyen, Emons, & Sijtsma, 2012). The choice between one scale or the other is therefore based on the amount of items, and on the different conceptualizations of both scales. The SGRAT-NL is based on three characteristics of individuals: lack of a sense of deprivation, simple appreciation, and appreciation of others; the GQ6-NL is based on four descriptive facets: intensity, frequency, span, and density."
I hope this contribution was valuable, I look forward to your findings.
Good luck and kind regards
Article Measuring Gratitude: A Comparative Validation of the Dutch G...