The correct answer is gerund; subject.
The verbal phrase we are talking about is finding the right travel companion, where finding is the verbal. The way it is written, it can either be a gerund or a participle because both of them end in -ing. However, since this is the subject of the sentence (What is crucial to an enjoyable trip?), it has to be a gerund because gerund act as nouns, and participles as adverbs/adjectives.