Biotest made a decision 2 years ago to sell many of their products exclusively through the T-Nation store. The idea was to cut out the middleman and therefore be able to sell at a lower price.
I think the figuring was that most of their serious customers were here anyway, so if they made their products with more attractive pricing, they'd wean us away from other brands.
Some products are sold at retailers because they are of interest to a more general population. Fahrenheit, TRIBEX "Blue" and HOT-ROX are examples. Some product are exclusive to T-Nation because they are sold at strengths that attract people more serious about working out (Alpha Male, Carbolin 19, Surge).
Don't know whether TRIBEX "Gold" will be here exclusively or appear in GNC.