So, you first have to wait for townhall to open up. Then, you need 50 rainbow stars for the 50 rainbow star chest in also townhall. You can get rainbow stars from townhall events or trading 7 yellow stars for one. You get 5 for winning a match of the event and 3 for losing. Then, when you get the key, go to sewers and when you go down the ramp where rux is, there should be a opening. So, go in there and you should find a big collection of gnomes and a locked gate. use the key to open the gate.
Then, open the 50 chest 4 times to get all 4 keys for the 4 trails. Your going to want to take a screenshot of the symbols it shows you at the end. Input the symbols where you hop into the blue portal in the middle when you enter the gnome portal and put in the code. Do it for all 4 gates and when you defeat the color guards, it should show 2 missing symbols for the code in each of the 3 portals. Put in the missing codes in the correct order to fight off the rainbow gnome king and then the big gold gnome should break open. and then you need to have 75 stars for each chest that contains torch and hover goat and there you have it.
Note: Townhall opens every 2nd Sunday of the month.