Good question. Let me break down the sentence for you.
First, "Jules Verne" is the subject of the sentence. Second, "exercised" is the main verb of the sentence, as it shows the action of the subject.
Third, "unique ability" is the direct object, because it receives the action. This might be a bit tricky, because it is abstract.
Now this is where it gets grammatically difficult. The word, "envisaging" is confusing to many people, because it could be both a present participle and a gerund. The way you can tell is by looking at the "ing" word and seeing if it acts like a noun. If it functions as a nouns, then it is a gerund. If it is more of a verbal adjective, then it is a present participle. In this case, it is a present participle and its antecedent is Jules Verne.
To make this clearer, you can always rewrite the sentence.
"Jules Verne, envisaging scientific inventions of the future, exercised a unique ability." The simple change of word order makes the participle clear. So, to answer you question, there is no gerund phrase; it is a participle.