What is the noun of thinking?

The Word 'thinking' is a noun called a gerund, the present participle of the verb to think that functions as a noun.

The present participle of the verb also functions as an adjective.

Examples:

I need a quiet place for thinking. (gerund, object of the preposition 'for')

I was thinking about taking a trip. (verb)

He writes thinking articles about current events. (adjective)