Just like you would merge any other mergeable item: You need to have the Forge built in Kingdom then you go in the Castle build screen and select a stack of 3 of same tier Garrison in the Inventory and activate the merge from the merge button on top of the screen, be sure you removed from your castle and put them into the inventory if necessary to do so (i.e. you had 2 in inventory and 1 was put into current castle). About gold we’ll add more ways to earn it later on