Qual è la differenza tra Gaan e Zullen para formar el futuro ?Sentiti libero di rispondere con un esempio.

Both 'gaan' and 'zullen' are used as auxiliary verbs of the future tense.
'gaan' can be translated as 'going to' in English, or as a form of 'ir' in Spanish.
'zullen' can be translated as English 'will' or 'shall', or the futuro form of the verb in Spanish.
Compare:
* Ik ga je helpen
- I'm going to help you
- Te voy a ayudar

* Ik zal je helpen
- I'll help you
- Te ayudaré

Apart from that 'gaan' is also a full verb, meaning 'to go'/'ir'.
Obviously this use can identified when there's only one verb in the sentence
* Ik ga (❌zal) naar huis/school
- I'm going home/to school
- Voy a casa/la escuela

Traditionally 'gaan' as an auxiliary is used when:
1. The subject of the sentence moves
to somewhere to do something
* Ik ga morgen in (de) zee zwemmen
- I'm going to swim in the sea
tomorrow
- Mañana voy a nadar en el mar
2. When an action begins
(in which case 'beginnen' (to begin/
to start) can also be used)
* Het gaat/(begint) te regenen
- It's starting to rain
- Empieza a llover
3. When there is a plan, an action
or a change in (personal)
circumstances or a decision has
been made
* Morgen gaat het regenen
- Tomorrow it will rain
- Mañana lloverá
* Zij gaan trouwen
- They are going to be married
- Ellos se van a casar

It can be confusing for foreign language learners to distinguish between
* Het gaat regenen
- Empieza a llover
- (mañana) lloverá
The actual meaning of 'gaan' has to be understood from the context.

'Zullen' is used with other verbs and has a build in meaning of certainty and promise
* Wat er ook gebeurt, ik zal komen
- Whatever happens, I (certainly) will
come
- Pase lo que pase, yo vendré

More and more 'gaan' is used instead of 'zullen'. This is a ongoing process
which has advanced more in Belgian Dutch de than in Dutch as used in the Netherlands. Up to a point even where (in informal speech) 'gaan' is used twice in one sentence, both as full verb and as an auxiliary
* Zij gaan morgen naar Brussel gaan
(Only in Belgian Dutch)
- They will be going to Brussels
tomorrow
- Ellos irán mañana a Bruselas

The following video (with English subtitles available ) might interest you
https://youtu.be/wjXtFr62DC4