Gunn was put in charge at a very bad time. The DCEU was in its death throes and needed a mercy killing/reboot, but there were also a full years worth of movies coming out and hundreds of millions of dollars on the line. Gunn had to announce a reboot and try not to inadvertently tank these very expensive projects. It clearly didn’t work out, though I’m not sure if any of these flicks would have done well even without the change of leadership and the reboot announcement.
Not sure what he said to Gal that made he think a third Wonder Woman movie was in the works. I’m thinking he was vague and said something along the lines of wanting her to be a part of the inevitable Wonder Woman project in his DC Universe. This could be anything from producing to pitching a story.