Gerund before prepositions [closed]

I read a sentence like this

Negotiating for dummies

I know that a gerund is after a preposition, so why in this sentence a gerund "Negotiating" is before a preposition "for"

Thanks for reading

A gerund may follow a preposition -- we say it's the object of the preposition -- because a gerund has usage as a noun. And you'll find it in other places where nouns are required.

Subject: Negotiating is an important skill.
Direct Object: I hate negotiating.
Nominative Complement: That was some negotiating!

But a gerund is also a verb (it's the present participle, the inflected form made by adding -ing to the plain form of the verb). And so it is associated with the syntactic roles that verbs have. Consider the gerund clause

I watched him giving a man money for coffee.

There's a subject (him), a direct object (money), an indirect object (a man), and an adverbial prepositional phrase of purpose (for coffee). These are all roles associated with verbs.