Bottom line, after reaching stage 50 (you will probably have already finished the quizes) the only way to get gems is through leveling up every character. Once you've done it the game is over, unless you're rich and don't care to pay to win. That's very discouraging!
Well you could still trade with card trader. You could still get the crummy cards from npcs and convert them. Though you probably only get a few cards a week so it'll be much slower. You'll probably want to save some gems for when new packs come out then.
Well, actually every game comes to this point and they *NEED* people to buy gems to keep the game going. They have employees and servers in their payroll. You can farm LD lvl 40 for gems. U get gems also for skill conversion. If u dont like the game as it is, u can leave. If u like the game, u can buy gems or farm the hard way. Your choice.
Except the face that they keep adding characters and each character can give 2000ish gold after max level. Also there are so many events that give out gems including special events like Pegasus and KC cup. Whiny babies can only cry though.
It is not really a p2w game if you spend your gem wisely. There is few decks, though not many, requires you to have multiple copies of UR cards in boxes. If you focus on 1 ~ 2 decks with free gems you can earn in game, you would be able to reach KOG. Anyway, if you want to have multiple competitive decks running under your account, you have to pay money.