Played every person I met and bought (or so I thought) every card in the game before finally doing a certain level 26 side quest revolving around Gwent. Before doing said side quest, I had a look at one of the various lists published on the interwebs (found
this one to be quite good) and made a comparison. As it turned out, I was missing nothing but leader cards - four of which were to be earned through the side mission in question - and the fifth and final card I was missing could be bought from a merchant. It wasn't too hard because innkeepers, merchants and blacksmiths give you a random card from a pool of cards. Once you have all the cards from that pool you just stop gaining new ones. The only other ways to get new cards are Gwent side quests and buying them from innkeepers. You basically cannot miss side quests and innkeepers are usually easily accessed through fast travel.