Just out of curiosity, why would you even care? If you want to eat healthy, start with the most influential and obvious and simple aspects (e. g. avoid sweets, sugar and trans fats in the first place; prepare properly or avoid foods rich in anti-nutrients like grains and beans). This whole computational nutrition fashion is in my opinion ridiculous in general, it mainly serves to advertise high-tech food-related products, rather than to provide benefit to the consumer.

From experimental experience I can assure you that in general, low GL diets cannot remedy acne. To generalize my self-experimentation (formerly a severe case, long-term observation): The major of acne cause are cleaning products (tensids); it may be possible that food incompatibilites (like gluten) also play a role for some people (hypothetical).

I have taken those measures and have seen improvement. I care, because I hypothesize that by switching to foods which have a lower GL, my acne will be less triggered. Numbers are important to establish a baseline. I have never seen the advertisement of high-tech food-related products when looking at glycemic index/load, and there's quite a bit of research that details the relationship between acne and diet. I prefer not to go over those things here.

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I prefer not to go over those things here.

This is sad, you could profit. Is there a reason why you do not want to discuss?

Nevertheless, to answer your question: Yes, add the GL numbers. The GI is so to say a food-specific weighting factor in this equation.
The result cannot be the higher of the GL, because the GL increases with the amount of food. The GL equals the GI times the total carbohydrate content of the serving size. A much more detailed table can be found in the supplemental material of this paper: http://care.diabetesjournals.org/content/31/12/2281/

Please keep in mind that the GI values are very uncertain in the literature and become more so when you consider the water content, carbohydrate content, serving size, preparation variation etc. Also, glucose and insulin levels may also depend on other factors like meal frequency. That means your approach does not become more scientific if you play with such inexact numbers (with unknown uncertainty limits by the way).

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WHY DO GI VALUES FOR THE SAME TYPES OF FOODS SOMETIMES VARY?

Many people have raised concerns about the variation in published GI values for apparently similar foods. This variation may reflect both methodologic factors and true differences in the physical and chemical characteristics of the foods. One possibility is that 2 similar foods may have different ingredients or may have been processed with a different method, resulting in significant differences in the rate of carbohydrate digestion and hence the GI value. [...much more of relevant information in the source]
Source: http://ajcn.nutrition.org/content/76/1/5.full.

 

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I have never seen the advertisement of high-tech food-related products when looking at glycemic index/load

I spoke more generally, but to my own surprise, indeed the GI already is all about money:

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In Australia, official dietary guidelines for healthy elderly people specifically recommend the consumption of low-GI cereal foods for good health (3), and a GI trademark certification program is in place to put GI values on food labels as a means of helping consumers to select low-GI foods (4). Commercial GI testing of foods for the food industry is currently conducted by many laboratories around the world, including our own. Many recent popular diet books contain extensive lists of the GI values of individual foods or advocate the consumption of low-GI, carbohydrate-rich foods for weight control and good health (5).
Source: http://ajcn.nutrition.org/content/76/1/5.full.

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I have taken those measures and have seen improvement.

I used to believe the same when I tried this. But note that the expectancy bias can have a strong influence, because acne
tends to vary a lot (hence the term breakout). However, actual improvement beyond expectancy biases is comprehensible,
because reduced glycemic load reduces sebum outflow and sebum composition:

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Dietary factors have long been implicated in the pathogenesis of acne [1]and[2]. It is well known that increased sebum production plays a fundamental role in acne [3] and evidence suggests that dietary manipulation alters sebaceous gland output. Extreme caloric restriction dramatically decreases the sebum excretion rate and these changes can be reversed when a normal diet is resumed [4]and[5]. Other studies have demonstrated that increased consumption of dietary fat or carbohydrate increases sebum production [6], and modifications to the type of carbohydrate can also alter sebum composition [7]and[8]. Altogether, these studies suggest that the quantity and composition of foods, when changed significantly, may affect underlying mechanisms involved in sebum production.
Source: http://dx.doi.org/10.1016/j.jdermsci.2007.11.005

Another reason why low-carb diets may alleviate acne is indicated by case studies of people with food incompabilities [ http://www.centerforfoodallergies.com/celiac_disease.htm, http://ndnr.com/dermatology/inflammatory-acne-case-studies-of-gluten-induced-dermatoses/ ]
This would also explain why acne was, to my knowledge, absent in primitive cultures, but present in civilized ancient ones who ate grains [ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2081863/pdf/procrsmed00581-0015.pdf, http://applications.emro.who.int/imemrf/J_Pak_Assoc_Dermatol/J_Pak_Assoc_Dermatol_2013_23_3_315_319.pdf ], where no harmful skin product were present.

In view of studies that demonstrate acne improvement on low-carb diets, I can understand your hypothesis. [e. g. https://bmcdermatol.biomedcentral.com/articles/10.1186/1471-5945-12-13 , http://ajcn.nutrition.org/content/86/1/107.short ]
But high sebum outflow is not the single *cause* of clogged pores. Sebum is still pouring out of my pores, but does not form new comedones any more.

Yes, avoiding high glycemic load meals, drinks & snacks will help acne & almost everything else to do with your health. But there's no need to calculate anything. Just eat mostly real, whole nutrient rich foods & never have empty high GL foods by themselves. Take a fiber capsule or have a handful of almonds or something when having a small cheat like the office birthday cake.

Also, it might be helpful to note that starches become resistant after they've been cooked & cooled. So pre-cooking your potatoes, oatmeal or pasta lowers the impact. Also, how much they are cooked & how. Don't overcook pasta.

You sound like me calculating my diet like the last resort. But probably it does not matter that much what you eat, as long you get all the nutrition, more the better, also try eating many different things, you can check the diet much easier, it helps or it doesn't.