Often it depends on how the band views Gothic as to whether or not they will consider themselves Gothic. Some bands will say that they are creating completely original music that defies classification, or that they do not wish to label themselves. Nonetheless, music reviewers and marketers like to classify music to help people get the feel for what the music sounds like for those who have never heard it before. It also helps them sell music to their target market. There is a considerable amount of flexibility when classifying a band as one thing or another.
Different people can put the same music into different classifications.
A band might change their sound or style from album to album with a variety of labels put on their various works. There's a good amount of crossover of music types as well, such as Gothic Industrial, when the same music falls into more than one category at the same time.