The wikipedia entry states:

'Bubble gum is available in many colors and flavors. A "bubblegum flavor" is the taste of the unflavored gum, made from chemicals such as ethyl methylphenylglycidate, isoamyl acetate, fruit extracts and others, although the exact ingredients were kept a mystery to customers.2 When blended, the chemicals and extracts fuse to make a sweet, palatable flavor. Gums made with vanilla, coconut, peppermint and almond extracts are available.'

Then goes on to list a range of specific flavours it is available in. While gum and bubble gum are available in many flavors, they also have taste of their own, which may be indistinguishable to many, and differ between brand, which perhaps makes "bubble gum" flavor a needless duplication of "Plain unflavored" above.

Update: fixed typo.