Here's a list of the sorts of things you find in most, if not all, Gothic fiction after Walpole--although it's not exhaustive, and I took out the things that come up later.

unexpected deaths
virtuous young peasant men
orphaned young women--usually they lost their mother young and have lost their fathers more recently (like Isabella)
tyrannts
speaking portraits
ghosts
prophesies
omens
lots and lots of guilt
divided loyalties
moonlight
lanterns being extinguished (especially by unexpected gusts of wind)
blasts of wind that rattle doors or windows
rain
storms that reveal psychological states of turmoil
exotic settings: often France or Italian, sometimes Spain
rusty hinges
slamming doors
curdling blood
trapdoors
underground passages
terror
gloom
anger
shock
fainting/swooning
Women persecuted/threatened by "dark" men (often virgins threatened with unwanted marriage, like Isabella)

medieval elements:
castles
castle vaults
convents (sometimes monasteries)