You can use salted or unsalted butter. If using salted, I would suggest omitting the garlic salt.
Be sure to gently squeeze out any excess liquid from the fish using a paper towel, especially if the fish was previously frozen. Too much liquid will cause the fish to steam, not brown.
Cook to an internal temperature of 140F using an instant-read thermometer inserted into the thickest portion of the filet.
Optionally top it with a lemon-butter Beurre blanc sauce shown in our baked Chilean sea bass recipe to give it that extra oomph.