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It’s still a diet, but for clarity you should call it a weight gain diet:

A weight gain diet is very similar to a healthy weight loss diet. In both cases, you will eat foods that are rich in nutrients and not eliminate major food groups. You will avoid "empty calorie" foods (junk foods that contain sugar, salt, and fat, but few other nutrients)

What Is a Weight Gaining Diet?

What's good about "weight gain diet" is that it works no matter who's doing it or why (even if that someone is an animal). It's also neutral and works no matter the level of formality.

In the fitness/bodybuilding context, the word bulk is used (the opposite process is a cut).

According to the OLE,

to bulk something out/up: to make something bigger, thicker or heavier

PS: I'm not sure this can be used in the requested form "I will go on a bulk", although an example can be found here.

Well, it is also called Ectomorph diet. Diet for a skinny person to become strong. Ectomorph means a person with a slim physique. So, ectomorph diet is the diet for slim person to gain weight.

Citation 1: Men's Body Sculpting By Nick Evans

An ectomorph's diet should contain a surplus of calories from complex carbohydrate foods to encourage anabolic weight gain.

Reference 1: Ectomorph Diet Plan Principles

Well, binging would be bad. Stuffing one's face would be bad. Even if one is underweight.

So, better is: I will increase my caloric intake.

Everyday speech: I will eat more (food).

There is no "going on an x" for eating more food.

This answer is for spoken English register.

Just for the heck of it, I'll answer the title of the question as asked instead of what the asker intended.

The opposite of "a diet" (choosing specific foods for your diet) is to not diet or to have "No Preferred diet".

So assuming diet is used to mean a program of constrained eating, the opposite would be unrestrained eating. Here are some possibilities:

indulge, over-indulge, over-eat, feast, gorge, binge, stuff, chow down, go to town

Some words that are near opposites, but of a different tense:

gluttony, voraciousness, gourmand

If none of those work you could take a word from Georgian. Shemomechama, literally “I accidentally ate the whole thing,” describing the incapacity to let the delicious food go to waste.

https://www.ozy.com/acumen/theres-a-word-for-your-thanksgiving-gluttony/74075

Actually, the most basic meaning of the word "diet" doesn't have an opposite... Proposed words like "gluttony", "overeat", "binge" are all very unhealthy concepts, and are not the kind of word you are looking for. In my opinion the best thing you could say if you are underweight is: "I need to change my diet and eat more calorie-dense foods because I need to gain weight..." Or "I need to change my diet in order to gain weight." Or more simply "I need to eat more calorie-dense foods in order to gain weight". You could also say "I need to go on a (special) diet to increase to gain weight".

Diet (noun): the food and drink usually eaten or drunk by a person or group (Source: https://dictionary.cambridge.org/dictionary/english/diet)

If you say "I will go on a diet", people will assume you will eat less, because of the second meaning of the word (an eating plan in which someone eats less food, or only particular types of food, because they want to become thinner or for medical reasons - same source as above).

In fact, there are some synonyms of "anti-diet".

I would recommend

gluttony

NOUN

mass noun

Habitual greed or excess in eating.

‘she said plumpness was a sign of gluttony in most cases’

(https://en.oxforddictionaries.com/definition/gluttony)

Here I am considering the definition of diet as: eating systematically in a controlled and restricted manner.

I would recommend, overeat.

However a quick search on google give me this results,

• binge


• indulgence

So the whole sentence with the three provided words would go like this

"I need to go on a binge (diet)."

"I need to go on an indulgence (diet)."

"I need to go on an overeating (diet)."

• If you are overweight you may say "I will go on a diet".


• If you are underweight you could say "I need to go on a ..."

diet. When i was a little kid i didn't know much about a diet. But nowadays i use two and i exercise a lot. Im always on a diet, when i'm overweight on early spring to late summer. I like to cut down my caloric intake. My second diet is from fall to late winter when i eat a lot and it makes me happy. Diet (healthy or not) is always here and i cant find the opposite.