I'm not seeing a "full explanation" that's still relevant -- there are some, but they are from years ago and a lot has changed. As I mentioned in the game hall, the merchant is a relic from ages gone by. I'd like to see it return, but to answer your question there's no real use of the merchant or gold on Gemp. You can get "foiled" cards and some alternate images can be gained from packs, but that's really not worth worrying about.
For leagues, all you have to do is pick a league and sign up for it. Go to the "Leagues" tab at the top, then "See details" under the league you're interested in. At the bottom above the league standings there's a "Join league" button. If this is a draft league, you'll need to leave here and come back; the "Join league" button will be replaced by a "Go to draft" button where you'll pick your cards. Afterwards go to the deckbuilder to create your league deck (if this is a constructed league, skip this step). In the middle at the top is a picklist that says "All cards," change that to the name of the league you're making a deck for. This will restrict the deck filter to only show cards that you have for this league. When you create a table in the game hall, select the league name at the very bottom of the list (below "The Hobbit").
Tournaments are something slightly different in Gemp, the tournament tables are down at the bottom. There are two kinds: on-demand and daily. The daily tournaments rotate between Fellowship and Movie each day and there are two per day, one at 19:30 server time and one at 00:30 server time. Both (currently, at least) require 4 players to start. For the daily, if there aren't at least 4 people in the queue at tournament start time then the tournament doesn't fire and everyone is kicked out of the queue. On-demand tournaments fire immediately when the fourth person joins, whenever that is. The difference between tournaments and leagues is that in leagues players are free to alter their decks however often they wish. In tournaments, the deck is locked in from the start and the decklist is shared with anyone who wants to see it under the "Tournaments" tab.
I think that about covers the basics of it!