You've really got it the wrong way round. A Skyline is basically a Japanese 3 series. Always had been. GTR and GTST just bought the name to the attention of the west, and the large majority of people don't seem to realise that to the Japanese, the word Skyline is nothing special. It's like saying that all 3 series are a disgrace because they're not an m3.
Hence why Nissan made the distinction with the R35GTR and dropped the Skyline bit.