It's not opacity, though I don't know what it looks like in your image program. It's the shade of grey used in the alpha channel of the normal map, not the RGB channels of the normal map. The alpha channel is a map with 256 levels of grey, and wherever you paint a lighter shade, that part is shinier. (White is shiniest, black is dullest.)
However, that alone will do nothing unless the model itself is set to use specularity in Gmax or other programs. When you import an MDB into Gmax, it sets the specularity to 0, no matter what it was before. For most models, I set it to 100%, with a glossiness of 20, and control the actual strength with the alpha channel of the normal map (it should be a proper map, not just a flood-filled channel, BTW). For particularly shiny objects, like the shiny floor in my more recent variety pack post, or the pillar you wanted, I use a specular strength above 100%.