Goblin Tunneler

The power of the targeted creature is checked both as you target it and as the ability resolves. After the ability resolves, the creature can’t be blocked that turn even if its power becomes greater than 2.

The ability doesn’t grant an ability to the targeted creature. Rather, it affects the game rules and states something that’s now true about that creature. After the ability resolves, the creature can’t be blocked that turn even if it loses all abilities.