You can get the 150 gems you need very easily if you know how. Here are some ways:

- do guild chests (chance for 10 gems)
- score high in the lab (chance to pick 10 gems)
- rank up slowly in the arena to grab gem stakes from others (this is how you'll get most)
- get 5-10 gems per sail by sailing on saturday
- get the next level reward
- save up the daily 10 gems you get from the assistant