In EU it's definitely gambling but so is every deposit you make. You put money in EU in the hopes that you will get back more than you put in.
But if you just look at it for face value though I would say the boxes you buy in the shop aren't so much gambling as just a more efficient way of depositing for the most bang for your buck. After all you are guaranteed to get the exact tt value you put in for those boxes as a minimum. Anything else is just a bonus.
The seasonal and special boxes however, since you have to pay a huge markup 40% to 500% of the min output value is totally a gamble. The only way to break even is if you get lucky. The only reason people pay the big markups on those boxes is in the hopes of looting the big valuable top prize (seasonal ring, ul amp...etc)