Answer:
If you remove parentheses your sentence should be alright. If it lacks logical ending/meaning then You have put the parentheses in wrong place.
A gerund is a word which takes the verb form -ing, but functions as a noun. The gerund in the given sentence is the word "collecting". The gerund in this sentence is used as a direct object. A direct object is a noun, that directly receives the action of the transitive verb and usually answers the question "what?". In this sentence, John has finally lost interest in what? The answer is collecting stamps.
