I bought "Hyper Light Drifter" on GOG under GNU/Linux (current version 2.8.0.9 - internally 5/31/2017)
to play it in coop mode.

Unfortunately we encountered the following bugs:
   o Freezing game without any specific reason (in Single Player mode)
   o Two Logitech Gamepads F310 not supported - the second controller is always totally ignored
      In a GOG thread (problem with DS4 button mapping) it was speculated that Linux is lacking concerning controller
      support and hinted to the later version for Windows 7/21/17.
      And after changelog July 2017 brought very important fixes for controller, so this update for Linux would be important:
      https://www.gog.com/forum/hyper_light_drifter/changelog/post13
   o The RS (Right Analogue Stick) is unused - would be great if this could be used for aiming
      (at least as an option to make it a real twin stick shooter).
   o It would be nice if the keys of keyboard and controller can be assigned - especially directions with keyboard
      should be done by cursor keys (the usual thing on PC since the 1980ies).
   o Is a 64 bit version possible/planned in future. Ubuntu wants to further restrict their 32 bit library support - so
      32 bit may cause severe problems under Linux in future.
      AFAIK it would need GMS v2 to export 64 bit binaries - or are some 32 bit libs used which are not available for 64 bit?
      It would be suitable for such a good and popular game to be preserved for future players.

I sent this error report yesterday (23.05.2021) to the devs and hope that while being busy with their new game will have
a look and may provide a new version which works.
Otherwise I will have to ask GOG for a refund - as I was hooked on locCoop and am not interested in SiPl mode here.

Addendum:

when trying not to use 2 controllers but KBM and one controller, one may see two avatars,
but both are controlled by the controller - so it is unplayable.
Additionally, it is clear that the program - after so many years - is still not working with old HW.

So if anyone else would want to play co-op under Linux, there are a lot of other games,
but this game is in current state no locCoop game.