Killing gostic does not prevent progress you must make sure all sub quests have progressed for them all to be at the throne or killed so their sub quest is considred finished. You must speak to Margott after defeating him at the throne in the capital, if Nepheli Loux is alive and you have given her the hawk then you must speak to Giddion to "get her back on her feet" causing her to leave the hold. at this point resting at the Godrick the Grafted grace will update their locations to the thrown. All NPC must be at the point for the movement for it to update. Alternativly if Nephili is dead for any reason but Gostic is alive then make sure his parts are progressed to be at the throne.

If u kill Gostoc, u miss out on a 2nd dragon Smith stone