Dead Cells, GOG Version, Xbox One Controller Support
By default, the GOG Version of Dead Cells ships with an older version of libSDL2 that does not support Xbox One controllers.
Fortunately, Debian Stretch does. Here is a simple way to force Dead Cells to use Debian's version of libSDL.
First, make sure that libSDL is installed
sudo apt install libsdl2-2.0-0
Then delete the game's shipped version (this is assuming that it is installed in the default location)
rm ~/GOG\ Games/Dead\ Cells/game/libSDL2-2.0.so.0
Dead Cells should then recognize the Xbox One controller without issue.