After buying GloomHaven I tried to download the game but it stops at 90%. I've got enought space on my hdd, no firewall rules block the download process, I tried to reboot my pc, reboot GOG, cancel then download again but nothing solve the problem...
I searched information on forums but nothing like my case is mentionnned.

Same issue here. I've tried restarting my computer, refreshing the download queue (by closing Galaxy and deleting C:\ProgramData\GOG.com\Galaxy\storage\jobs.db), giving Galaxy an exception in my firewall settings, and even starting the download over again. It always gets stuck at 9.33 out of 9.58 GB. Other downloads/updates through Galaxy are working fine, so whatever the issue is, it seems specific to Gloomhaven.

I had the same problem a couple days ago. Tried my luck again today and it worked, so maybe it'll work for you now too?