Will be focusing on the texts of Frankenstein, Wuthering Heights and Doctor Faustus in this answer.
Introduction - explain what exactly is meant by 'transgression' in the context of the Gothic genre - i.e. going beyond boundaries of nature/society/science/culture/academia/the physical world. Can be construed very broadly, so stick to one or two elements which are relevant to your texts and focus on these. Set out your argument clearly in the introduction so the examiner knows where you're going with your answer. You will likely have something like an hour to write on three texts, so to account for reading and planning time of 15 minutes, aim to spend around 15 minutes on each text. Try and give equal time and attention to each. One key thing to note is that this is a GENRE question - it talks about the GOTHIC - so always link it back to this aspect.
First paragraph - Frankenstein - explain how titular character transgresses science and the boundaries of nature by creating the monster. This is Gothic because it usurps regular notions of the self and what it means to be human, which makes us feel uncomfortable and is therefore transgressive
Second paragraph - Wuthering Heights - explain how Cathy and Heathcliff's love transgresses the boundaries of nature and the physical world - 'Whatever our souls are made of, his and mine are the same' - lots of spiritual language. Could also be said to transgress boundaries of the person - 'I AM Heathcliff' - they effectively become one person through their love for one another. This is Gothic because it taps into an intensity which is unseen and intangible in the regular physical world and has echoes of the sublime and the unknown
Third paragraph - Doctor Faustus - explain how Faustus transgresses the boundaries of academia and the expectations of man - hubristic, Promethean, Icarean, his ambition (greed?) is eventually his downfall. Draw cross-textual references with Frankenstein here for extra marks. This is Gothic because it shows an excess which ultimately ends in death - a gory and excessive end.