You have so much choice:

For the pure fat experience I suggest corticosteroids, lithium or a random antipsychotic. Insulin is also very useful here, but a tad dangerous.

If it has to be muscle mass, then anabolic steroids of course are preferable. But beta adrenergic agonists like clenbuterol also seem to have some effect.

And there are many more.

I don't want you to consider it among the flippant replies here but definitely alcohol will work up to a point, it's somewhat of a contradiction though because severe alcoholics are usually quite malnourished and thin.

Its called sugar

Steroids and antipsychotics usually cause massive weight gain.

Weight from muscle? Or from fat?

Ice cream and cheese

mj

Antipsychotics tend to do the trick.

Gaining weight works by putting more fuel into your body than you use. That's physically the only way, there is no way to synthesize energy. Because of caloric density limits, there is no substitute for putting a certain amount of stuff in your body. A pill can physically not contain enough calories.

There are options around that, but that depends on what's driving your personal energy deficit. What's the job that the weight gain intervention needs to do for you, that whatever you try to eat right now does not? What's motivating your current diet? Are you for example disgusted by swallowing anything at all? Are you more partial to liquids than whole foods? Do you dislike sugar in particular?