But I was just interested in why the GNU prolog was not the "most popular"[1](https://github.com/geany/geany/pull/3171#user-content-fn-1-9a210e28c532bc078...) open prolog, and like you I used @rdipardo as the most prolog savvy person here[2](https://github.com/geany/geany/pull/3171#user-content-fn-2-9a210e28c532bc078...), so asked him.
It was SWI that I used back at school so it was "most popular" for me :-). But judging from things like the number of stars (the true and only measure of quality, popularity etc.) at github, number of contributors (GNU prolog seems to be more or less a one-man-show) and commit frequency, SWI seems to win.
https://github.com/SWI-Prolog/swipl-devel https://github.com/didoudiaz/gprolog