The chests in this game is gambling, usually illegal in countries until your about 20 years old. Just like gambling is a bad idea so is buying chests unless your willing to throw your money away without getting much in return.
Except this time you always win something so technically no.
When you gamble and lose it doesn't mean you don't win anything, it means you get back less than what spent. While a lot of gambling games are all or nothing each round, over the course of time you play you will win and lose some and overall you will have either won or lost depending on if you break even. Like in poker or slots you cant win less than you put in on that round unless you get nothing, but there are some games (if you've ever heard of Rummoli) where you can win less (but still something unlike a round of poker) than you put in during that round.
The best argument for chests not being gambling is the fact that there is no chance of you actually getting anything of real value from the chests. Your leasing virtual items or essentially an inconsequential amount of storage space to keep some data on VR's servers. No matter what you win you are always getting the same thing which is worthless and VR is only loaning you the items, it doesn't belong to you.
I would argue it is gambling for this reason. When using chests there is no guarantee that you will ever get a specific item. Take any single item in the chest drop table and it is possible that it never ever drops from a chest. In this way, it is always a gamble which item you will get.
Another aspect that makes chests more similar to regular gambling is that it's possible to trade any item gained from chests. This means that there is a way to acquire items from chests without the randomness factor. Just like irl where you can always get something without having to gamble for it. The thing gambling does is randomly changes the time or money it might take to acquire it. Buying chests just randomly changes the time and money it takes to get say a dragon egg but given a certain amount of time and/or gold you can obtain it from other players. This doesn't change whether or not it's gambling it just makes it more similar to gambling IRL.
Either way, whether you always win something or not is irrelevant when arguing when something becomes gambling. Its the randomness factor and the possibility of getting less than what your money was worth.