Hi Kapvijay, and welcome to the AL forum!
Here 'having' is verbal participle, isn't it?. So How does the preposition 'for' take verbal participle instead of noun?
The most important thing is that at least it isn't
...for *have sent me the brochure LOL. That is, '-ing forms' (nice fuzzy indeterminate term, that even long-winded grammarians like Huddleston & Pullum favour, though they call it 'gerund-participle') are the norm when you need a "verb" after prepositions, and a decent grammar ought to tell you in no uncertain terms that prepositional phrases aren't composed only of preposition + strictly noun. For example, the following is from the 'prepositional phrase' entry in Leech's
An A-Z of English Grammar & Usage:
Forms of prepositional phrases
most common: PREPOSITION + i) NOUN PHRASE or ii) PROUNOUN
less common: PREPOSITION + iii) -ING CLAUSE iv) WH- CLAUSE v) ADVERB
In his
A Glossary of English Grammar, Leech calls such -ing clauses following prepositions 'prepositional complements', which is basically the same thing as an 'object of a preposition'.
Then, here is Swan in his
Practical English Usage (Second edition, entry 406.6):
Note that -ing forms after prepositions can often be considered as either participles or gerunds - the dividing line is not clear (see 290).
"Bonus": 1) From the perspective of a British English speaker (moi), the perfect aspect in the -ing clause seems unnecessary (cf. ...for sending me the brochure) and might be one reason for your interpreting the clause as more verby. 2) Although 'thank' can be a verb, the ellipted Present progressive form in your original sentence (
(I'm) Thanking you...) sounds too literal and thus strangely direct to my ear. (My instinct would be to soften things with "distancing" past tense, and perhaps an adverb:
I just wanted to thank you for...). Or one could just use the interjectional (LongmanDoCE, Macmillan ED)/exclamatory (OxfordALD, CambridgeALD) and thus truly subjectless phrases 'Thank you for' or 'Thanks for' rather than a tensed verb.