re: Is there any chance we get a satisfying ending to GoT?

Gendry doesn't matter. There's two prominent big magic bloodlines in the series Stark (Ice) and Targaryen (Fire).

Baratheon has some Fire in it since a cousin of Aegon I dethroned the original storm king and since then the targs and baratheon commonly interwed with great grandparents of gendry providing a recent infusion.

Gendry's only point in the show was to alleviate casting and torturing a boy because all he's doing in the book is running an Inn/ with the brotherhood without banners but he and the boy both had the Fire.

Melisande is pretty point blank in both the show and books that Shadow (Ice) and Light (Fire) are the same thing in the Lord of Light.
Jon or Dany would be much better power sources for someone like her. Hell the Wall itself is a huge wellspring of magical energy.