The answer is dependent on the intention of the buyer, and this is why TPCi have been so successful at monetizing addiction through combining a quality IP with a hybrid version of gambling.
On the one hand, they promise to provide an exact number of cards for an exact value which is a promise fulfilled in 100% of transactions, assuming we ignore unintentional error packs. The buyer is therefore not gambling on the quantity of the product received. On the other hand, the cards are randomized and the Pokémon Company makes no guarantee of exactly which types and rarities of cards will be received through the purchase of each pack.
If one person buys Pokémon booster packs because they simply want to own as many cards as possible (by which I mean the literal number of cards overall, not the number of unique cards), then it cannot be defined as gambling, because the purchasing party has achieved their goal through the transaction; they bought 10 Pokémon cards. The randomized nature of the cards in this instance is irrelevant.
But. The majority of people obviously do not buy them with this goal in mind. They buy them in the hope to gain something of as high a collectible or monetary value as possible. Because hope and uncertainty is involved, TPCi have therefore facilitated gambling through the purchase of their product. In this instance, it meets the definition of gambling.
Because TPC do not dictate the desires and goals of their buyers, they can present one of the seemingly most innocent forms of gambling the world has seen, particularly as it’s primarily aimed at children. The reality is that this method of corporate enterprise is really quite insidious; TPC can claim that they do not dictate the desires and goals of their TCG buyers while inherently (and strongly) influencing it through the core design of the product.