There are multiple examples of the Gothic in Macbeth. Any reference to superstition is a Gothic reference. So, you can quote the witches at any point in the text to reference Macbeth as containing Gothic elements. Another example seen in Gothic literature is the setting of medieval times (castles, dungeons, etc.).
Also, the use of darkness and blood are also seen dominantly gothic literature. There are, again, many quotes that you can use to reference these as well.
That being said, perhaps one quote that stands out as exhibiting typical Gothic features (references to the supernatural and the typical Gothic setting of the castle) is one from the Porter:
Here's a knocking indeed! If a
man were porter of hell-gate, he should have
old turning the key.
[Knocking within] 750
knock, knock! Who's there, i' the name of
Beelzebub? Here's a farmer, that hanged
himself on the expectation of plenty: come in
time; have napkins enow about you; here 755
you'll sweat for't.
knock! Who's there, in the other devil's
name? Faith, here's an equivocator, that could 760
swear in both the scales against either scale;
who committed treason enough for God's sake,
yet could not equivocate to heaven: O, come
[Knocking within] 765
Here the Porter references the castle by acknowledging that he must come to answer the gate to callers. Also, the Porter references the Devil (a supernatural element).