A. Talha Yalta and A. Yasemin Yalta, "GRETL and its Numerical Accuracy",
Journal of Applied Econometrics, Vol. 22, No. 4, 2007, pp. 849-854.

The file gretl1.6.0-review-sup.pdf contains supplementary material. In
particular, it discusses the tests of numerical accuracy that were
performed and presents results in tabular form.