You can only unlock certain chests in Granblue Fantasy Relink when you have matching keys, and Gold Keys are just one type of Key you can find. Getting Gold Keys is a bit of an endeavor, though, as you need to finish a specific Side Quest.
Depending on where you are in Granblue Fantasy Relink, you might not even be able to get a Gold Key yet. Once you do get access to the proper Side Quest, bear in mind you’ll be tasked with taking down a Cobra, which can poison you if your party isn’t prepared.
How to get Gold Keys in Granblue Fantasy Relink
Once you get through Chapter 8 in the Granblue Fantasy Relink story, you should unlock the Out of Serpentine Spite Side Quest from an NPC called the Adventurer’s Partner in Seedhollow. On this Side Quest, you’re tasked with clearing out a single Cobra from any other quest to help inspire an adventurer who’s about to retire because they’re frustrated with the creature.
There are several ways you can meet this requirement, but the Rilla Done Did It quest, that you can claim from the Quest Counter, is only Normal difficulty. Any quest featuring a Cobra will do, but this is by far the simplest way to get your Gold Key, since you’ll blow through it with the right party.
Once you have your Gold Key, you can unlock any Gold Chest you stumble across. There are at least two chests located in Seedhollow specifically—one near the docks and another closer to the Seedhollow castle gates. You can open both using the Gold Key. You’ll also find Silver and Whitewing Keys on your journey that open their corresponding chests throughout the game.