You can't take Gwent with you. CD Projekt Red has laid out what will carry over into The Witcher 3's upcoming New Game + mode, and while your sweet new sword will make the leap just fine, your hard-won Yennefer card won't.
Other non-transferable items include books, letters, trophies, quest items, and "usable items," but at least your money and alchemy recipes will be right where you left them. If you haven't hit level 30 in your old game, you'll find your level boosted appropriately in New Game +, but otherwise your experience will carry over. You're free to play (and earn achievements for) any difficulty setting despite that potential boost.
Even Geralt's old skills will persist in New Game +, though you'll get a free Clearing Potion at the start if you want to respec into a different combat style. But before you rush through the end of Geralt's tale to start over anew, remember: even as you cut through drowners with super-powerful endgame gear and abilities, you'll still have to start over where it really counts: your painstakingly assembled Gwent deck.
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