I am grateful to what I have been bestowed (upon).
Is the preposition 'upon' a must? Thank you.
Do you mean "I am grateful for what has been bestowed upon me"?
That is, you're thankful for what you've been given.
If so, 'upon me' is a prepositional phrase, and must have a preposition.
"I am grateful for what has been bestowed" is not a good sentence, unless we already know what's been disposed, and upon whom.