Kosher vs. Sea Salt

I'd substitute via weight because sea salt and kosher vary in texture/size. I have a Norwegian friend whose grandma made gravlax for him in Norway and buried it in ice! You've got flexibility here.

Sounds like you've made gravlax before and know the right level of savoriness. How about finessing the curing mixture? Add your salt of choice and taste the mixture before putting it on the fish.

do i understand that you only have sea salt but the recipe calls for kosher salt, or the opposite? for gravlax, the salt is ultimately going to melt a bit into the fish, and after curing, be scraped away. So i don't see a problem subing equal weights or measurements; not a big dealbreaker in my mind.I LOVE gravlax. btw, it freezes very well so i always make more than i need. so many herbs/flavorings are too subtle for me, but martha stewart's Entertaining book has a great mint version (and i'm not even a big mint lover.) go figure. but it's the best gravlax i've evr had.
sugar,salt,cracked peppercorns, crushed mint leaves.

Sea salt usually has a significant amount of impurities, enough to taste in some recipes. It also costs more than what many chefs consider the standard, Diamond Crystal Kosher, which is cheap and neutral in flavor.

If the recipe gives you the weight to use and you own a kitchen scale (you should), you're good to go. Otherwise the problem is determining the amount to use as every sea salt and every kosher salt has a different size crystal and therefore a different weight for any given volume. You can try substituting one for one and make adjustments from there.