Will iPhone remain "water resistant" after gasket replacement?

Typically the liquid resistance shouldn’t be trusted after about a month from brand new. But to answer your question, I wouldn’t trust it. Removing the screen whether it was cracked or not will break the adhesive seal. Without the proper tools and equipment that only apple has, you won’t be able to seal it properly. I myself replace it in my shop but I inform my customers that although I replace the seal, just like Apple, that I do not guarantee it will hold up.