Learn Hardcore Portuguese (Portugal): Quando estou cansado, gosto de me sentar no banco no jardim. - When I am tired, I like to sit on the bench in the garden.

You cannot drop de here; gosto sentar is ungrammatical.
You also can’t say gosto me sentar because the pronoun has to be linked to a verb, and de must come between gosto and me sentar:

✔ gosto de me sentar
❌ gosto me sentar
❌ gosto sentar

Why is it me sentar and not just sentar? Do I really need me?

Yes, in European Portuguese sentar-se is normally reflexive when it means to sit down (to take a seat).

sentar alguém = to seat someone / make someone sit
sentar-se = to sit down (yourself)

So:

gosto de sentar crianças no banco – I like to seat children on the bench
gosto de me sentar no banco – I like to sit down on the bench (myself)

Without me, you would be saying that you like seating someone else, not yourself.

Could I also say gosto de sentar-me instead of gosto de me sentar?

Yes. Both are grammatically correct in European Portuguese:

gosto de me sentar no banco
gosto de sentar-me no banco

They mean the same thing.
In more formal or traditional grammar, placing the pronoun after the infinitive (sentar-me) is often preferred, but in everyday European Portuguese gosto de me sentar is extremely common and fully acceptable.

Why is there a comma after Quando estou cansado?

Quando estou cansado is a subordinate clause (a time clause: when I am tired).
In European Portuguese, when such a clause comes first in the sentence, it is normally followed by a comma:

Quando estou cansado, gosto de me sentar…
Se tiver tempo, vou ao cinema.

If the quando clause came after the main clause, you would normally omit the comma:

Gosto de me sentar no banco no jardim quando estou cansado.

Why is it estou cansado and not something like sou cansado?

Portuguese distinguishes between:

estar – temporary states, conditions, feelings
ser – permanent characteristics, identity, inherent qualities

Being tired is a temporary physical state, so you use estar:

estou cansado – I am tired (right now / at the moment)
sou cansado – sounds like “I am a tired person (by nature)”, and is unusual; you’d rarely say this.

If a woman is speaking, does the sentence change?

Only the adjective cansado changes to agree in gender:

Man: Quando estou cansado, gosto de me sentar no banco no jardim.
Woman: Quando estou cansada, gosto de me sentar no banco no jardim.

Everything else stays the same. Adjectives agree with the gender and number of the subject.

What exactly does no mean in no banco no jardim?

no is the contraction of the preposition em (in/on/at) + the masculine singular article o (the):

em + o = no → in the / on the / at the

So:

no banco – on the bench
no jardim – in the garden

Literally: on the bench in the garden.

Is it okay to repeat no twice: no banco no jardim? Could I say no banco do jardim instead?

Both are correct, but they differ slightly in nuance:

no banco no jardim
- Literally: on the bench in the garden
- Neutral: there is a bench, and that bench is located in the garden.
no banco do jardim
- Literally: on the garden’s bench / on the bench of the garden
- Emphasises that the bench somehow “belongs” to or is specifically associated with that garden.

In everyday speech, no banco no jardim is very natural and clear.

Does gosto de me sentar mean I like sitting or I like to sit? Is there a difference?

In this context, Portuguese gosto de me sentar can be translated both as:

I like to sit down
I like sitting down

English distinguishes to sit vs sitting, but Portuguese uses the infinitive (sentar) for both meanings. There is no difference in meaning here; you choose whichever English version sounds more natural in your context.

Why is it Quando estou cansado with the present tense, not something like a future tense (like in English “When I am tired, I will…”)?

Portuguese often uses the present tense after “quando” to talk about general or future situations:

Quando estou cansado, gosto de me sentar…
– When I am tired, I (usually) like to sit down…

English might use either present or future in the main clause (I like / I will like), but in Portuguese:

After quando (when), for habitual or general facts, you use the present:
- Quando chego a casa, tomo banho. – When I get home, I take a shower.

You’d only use a future tense after quando in more specific or formal contexts, and even then the present is extremely common in European Portuguese.

Could I say Quando eu estou cansado instead of Quando estou cansado?

Yes, both are correct:

Quando estou cansado…
Quando eu estou cansado…

In Portuguese, the subject pronoun (eu) is often dropped because the verb form estou already tells you the subject is I.
You might include eu for emphasis (for example, contrasting with someone else):

Quando eu estou cansado, gosto de me sentar, mas ele prefere caminhar.
– When I am tired, I like to sit down, but he prefers to walk.