When were you a powerlifter?? And why not just eat the same way you did back then?
The glycemic index is a relatively-old way of looking at bodybuilding nutrition. But anyway, yes, add carbs. And fats. And protein.
P.S. - Half of your daily protein should not be coming from protein powder, unless it's part of a bigass high calorie shake. Focus on a lot of whole food with some powder, not the other way around.
Re-read the article. It flat-out says "The following table gives you an estimate of what your caloric intake should be set at depending on your lean body weight
This caloric intake should allow you to gain around two to three pounds per month. If you aren't gaining that amount, slowly increase your caloric intake until you reach that rate of growth (add 250kcals at a time)."
If you're not gaining weight, eat more. Could not be a more simple concept. Overthinking it will slow you down.
That's part of it, sure. The scale should go up every week, but that's once you get everything dialed in.