I have found many allusions to this feud, e.g. from http://nickdunbar.net/2012/05/23/thinking-fast-and-slow-by-daniel-kahneman/ and in Kahneman's trade book "Thinking, Fast and Slow". However, I have not found any concise explanation of what Kahneman and Gigerenzer actually disagreed on, other than that Gigerenzer felt that some of the Kahneman/Tversky biases were artefacts of their method and Kahneman/Tversky disagreed with this.
I have been quite astonished by this nonsensical yet lasting quarrel. You didn't find how they disagree because they don't disagree. The sole difference is that if asked "are human rational ?", Gigerenzer answers "yes", Kahneman answers "no".
However, their model of human reasoning are consistent with each other. They just don't use the word "rational" in the same way.
When asked whether humans are rational, Gigerenzer understands the question as "Are human cognitive mecanisms fitted to the environment ?", whereas Kahneman understands this question as "Are human taking decisions that maximize their utility function ?". Kahneman study decisions, and he finds that they are not good/not efficient (i.e., not rational). Gigerenzer study reasoning mechanisms (his heuristics), and he argues that those mechanisms are good/efficient (i.e, rational). Decisions and mechanisms are not the same thing.
If you ask Gigerenzer whether humans take decisions that maximize their utility function, he will happily answer "No". Indeed, Gigerenzer wants people to take better decisions, but he will not call those "more rational decisions". If you ask Kahneman whether human cognitive mecanisms are adapted to the environment, he will happily answer "Obviously, because evolution is true". To me, Kahneman's definition of rationality makes much more sense.
For a more extended discussion, you can find the following paper which settles the question on google scholar : Samuels, R., Stich, S., & Bishop, M. (2012). Ending the Rationality Wars. Collected Papers, Volume 2: Knowledge, Rationality, and Morality, 1978-2010, 2, 191.