I use Costco salmon for gravlax all the time.
If there's dark meat on the fish it will be fishy...carve that stuff off.
I give it a good salting...sugaring...whatever spice...and put it dry into a zip lock bag...place it on a dish...put a flat bottom plate atop...place I big can San Marzano tomatoes atop that...in the fridge....turn daily .
Its never dry outside. Was it in plastic?