This is correct usage.
"Bob winning" is a subordinate clause serving as a NOUN PHRASE serving as the object of the preposition of "of".
Noun phrases can be simple adjective noun constructions or a clause like "Bob winning."
For example you could also say.
Bob winning is a good thing.
In this sentence, "Bob winning" is the subject of the sentence.
"Bob's winning" changes the syntax used making winning the direct noun of the sentence and making it simpler to understand syntactically, but that makes "winning" in and of itself a noun being modified by "Bob's"
This article explains noun phrases well.