I’d you decided to spend gems to pass the goal, don’t spend gem by clicking “skip”the goal. I don’t remember how much it was exactly but it costs somewhere around 200 gems. If you go to the workshop, you can buy a key for 2 gem and the key goes to your inventory so it will cost only 40 gems to complete the goal.
I was saving the keys for two days just to make sure goal 2 auto complete. Then I forgot about it and made two chest just before goal 2 this morning. I needed 6 more keys to complete the goal so I bought them for 12 gems in the workshop. I had to force close and restart the game for the keys to register in the goal, btw.