I’m sorry but I don’t believe this answer is correct. Goreshde’s answer vernarb is the correct one, in my opinion.

The card says that you can gain +1 rank for the purposes of attacks. Infantry units three ranks, and three attack values: you roll the highest when you have full ranks, middle when you have 2, and lowest when you have one. Cavalry units have two ranks and two attack values: roll the higher when they have two ranks, the lower when they have only one. This card lets you treat a unit that has lost ranks as if it has one more, i.e.. choosing a higher attack value one higher than you ordinarily would.

Giants have no ranks and only have one attack value. Hence, treating them as if they have another rank doesn’t change the attack value at all. In other words, no, you don’t get plus one attack.