Hi Raquel! The correct form is 'regret watching it'. There is no predictable rule for which verbs are followed by a gerund and which are followed by an infinitive (and some can take both!). It is more a question of either memorising or getting a 'feel' for what sounds right/wrong. Because regret is an emotion related to the past, it will always describe the past. But (like in your example) we can express that someone will in the future look back at the past and think a decision they made was wrong using "will/gonna" + "regret"