While I can't say anything officially, from my understanding of this game (basically limited to what I've read and the few games I've played on the alpha/beta tests).
Ways to obtain Gran:
- Mirage Master (Once a day, I'm assuming it picks the first win)
- Recycle cards in the shop
- Buying it with real money
Ways to obtain New Cards:
- Per level up
- Treasure Battles
Of course if you spend enough time playing you might take part in certain events which result in you getting cards "out of the ordinary" (battles with GMs and other such things, but those are unpredictable so are not in the lists). But like I mentioned, these are partially assumptions made from a few games and what I've read in the site/forums, so I'm probably missing/wrong about some things.