Looks like the same number of companies per graphics chip companies, NVidia and ATI are the same. Recently, after the CeBit show, ASUS said they'd stop manufacturing NVidia cards and would jump on ATI's Radeon bandwagon. Gigabyte, about a week ago, decided to leave ATI and join NVidia.
Source:
Giga-byte.com
Some of Gigabyte's offerings include Enthusiasts as well as mainstream cards. On the roadmap, it does not discuss anything about the 5900 Ultra or the 5950 Ultra.