I'm excited to see your painting!
It looks like Bob used some kind of brown, maybe Burnt Umber acrylic paint mixed with White Gesso to do the underpainting. I suspect any brown acrylic (e.g. umber, sienna) would suffice. In fact, you can also probably mix together any complementary colors to achieve a brown you can then mix with your gesso (i.e. red/green, blue/orange, yellow/purple).
I think Bob just forgot to mention that he applied Liquid Clear to the whole canvas (after the underpainting dried), before adding the Indian Yellow at the beginning of the episode. He mentions it later.
Hope that helps!