First of all, always read the disclaimer¹. I was watching an episode of Extra Credits a while ago and they were addressing loot boxes in games. At 4:22 in the video the question of whether loot boxes are gambling comes up, they say that more research is required but that the only study they could find found that

Both legally and psychologically, there's an important distinction between gambling and non gambling and that is the ability to cash out. Because you can't take your rare Overwatch skin and sell it back to Blizzard for actual spending money, the experience affects us differently. Video game loot boxes are less like craps and roulette and are more akin to a crane game, or a blind box, or the raffle for prizes at the county fair.

And I take issue with that², because I think that the way we define currency needs to change. The Oxford English Dictionary defines currency as:

A system of money in general use in a particular country.

But I think a better definition would be something along the lines of

Anything that is used to facilitation trade

In fact Extra credits did a brilliant series on The History of Paper Money where they discus things like large rocks that were used as currency.

All sorts of things have been used as currencies, there are stories of cigarettes being used as currency after World War II, even by people who don't smoke. Consider bitcoin, I would argue that bitcoin and other cryptocurrencies are currencies.

If you search online you will find plenty of stories of the gold in World of Warcraft (WoW gold) being worth more than some or other countries currency³, and people who make full time jobs farming WoW gold and then selling online to change it back to their local currency.

You could argue that WoW gold is not a "real" currency because you can only buy things in Wold of Warcraft with it, and to buy things outside you need to convert it into another currency, but I would say that is true to some degrees of all currencies.

Even if you take what I assume⁴ to be the most widely recognised and accepted currency, the US Dollar, you still can't use it to buy anything anywhere without converting it first. If you walk into McDonald's, the most American store I can think of, in Australia and try to buy a Big Mac with US Dollars you will be politely asked to go away and come back with real money. There is no currency that is universally accepted everywhere without the need to exchange it into another currency.

So, I believe that currencies in games, in particular games which allow players to trade with each other and therefore create an easy market for people to sell their in game items are "real" currencies. And by extension I believe extension loot boxes are gambling because you can cash out.