I'm not sure what the difference from Chromebook, but if it's more capable then Chromebook Pro would do.

If they want to compete with Apple then focus on Pixelbook, maybe (ditch other vendors).

Another option would be to buy Ultrabook trademark from Intel.

Yes, they could have done a chromebook or pixelbook variant. No sane reason for a third brand that dilutes the other two aside from internal google politics/promotions and to show shareholders.

This does nothing for the customer and you can see a brand called Apple which has been successful with the Air, pro and neo under one brand.

Chromebook Pro is the obviously better choice in your list. Googlebook is clearly the worst.

alphabook

gbook