Depends how you define gambling:
play games of chance for money? Not gambling
take risky action in the hope of a desired result? probably gambling
Lootboxes in Overwatch are currently a progression system where the order of progression is random.
When you open a loot box you are guaranteed a cosmetic unlock that has no intrinsic value - You cannot sell or redistribute the unlock - It merely adds unlock progression to your account.
Lootboxes in overwatch are a core part of the base game and it’s unlock methodology - Nothing in purchased lootboxes is of greater or less value than a regular unlocked loot box.
In many other games, loot boxes are available with unique rewards only to those who purchase them. In these cases loot boxes are gambling rather than accelerating a predefined progression. Especially when you can transfer the item, instead of being a progression system it becomes gambling as users can put in money with the intention of potentially getting something more valuable out.
Basically, Overwatch loot boxes are not gambling because buying them doesn’t give you a chance to get something more or less valuable. All loot box skins are effectively worth $0 as they are non-transferable and in over-abundance.
Some users however might believe the unlocks are more or less valuable and thus to them, they gamble.