Traditionally Gothic Architecture is defined as having three features: flying butresses, gothic arches and ribbed vaults. All of these features appeared in Romanesque Architecture at various points, but Gothic Architecture is unique in combining them.
When researching Gothic architecture try to find the date of construction for any buildings you wish to use as examples. This is because the Victorians (19th century) went through a period known as the "Gothic Revival" in which they tried to imitate this style of architecture but they never quite got it right. (The CBD of Melbourne, Australia, has many examples of gothic revival architecture.)