I'm so happy you asked this as I am in holistic nutrition school to learn this exact kind of stuff!
The more important thing that what to eat, is what to avoid.
Be very careful with the following (and ideally avoid all of it)
-Process food
-Food high in trans fat
-Foods high in sugar
The issue with sugar is that it will spike your blood sugar, making you hyper and then making you crash. It can also put your body into a 'fight or flight' state which inhibits your ability to think logically and rationally.
The processed foods have irritants in them which the body doesnt like and sometimes can't digest, thereby causing that same fight or flight mode and just making you a little sluggish.
The trans fats have a whole whack of issues and affect cell functioning negatively.
As for what to eat, it's good to mix protein and fats with your carbs to ensure a steady supply of energy rather than the quick and crash which comes from sweets.
Make sure you're getting a good amount of fibre and a good amount of movement so that your digestion and bowels are functioning properly. There's nothing worse than not going poop for days because you're not moving and because you're in a stress state.
In terms of supplements for focus, there are many. Green Tea is a solid clean energy and Gingko Biloba is great for short term boost (late in a tournament for example). Vitamin B12 taken sublingually is one of the best sources of energy I've found (and there's no toxicity sign so you can take as much as you want).
The key is always to not overdo it, and ensure you have that solid rest routine to keep your system stable. Online poker has a way of getting people into a very amped up state (especially if multitabling) and so having a cooldown routine like yoga or walking in the park is a great balance.
https://www.youtube.com/watch?v=5eUgjeyQlEc&t=214s
I'll have a lot more specifics in the future as I'm still early on in the program. But the main thing is combining your attention on both diet AND lifestyle. Diet gives you the building blocks for having high focus and good energy, but lifestyle, exercise, rest and all that is what enables your body to actually be able to make use of the building blocks you've provided it.