Bigger's fear of Gus stems from the plan to rob Blum's deli. As we learn early in the scene, Bigger is "fascinated with the idea of the robbery, and a little afraid of it."
However, Bigger refuses to show any fear to his friends. In fact, in presenting himself as unfraid of robbing Blum, he establishes his power over his friends and gloats about the possibilities. The plan, however, backfires when Bigger realizes that if Jack and G.H. agree to the robbery then Gus will too and Bigger would then have to own up to his own fears of robbing Blum.
Bigger was afraid of robbing a white man and he knew that Gus was afraid, too. Blum's store was small and Blum was alone, but Bigger could not think of robbing him without being flanked by his three pals. But even with his pals he was afraid. He had argued all of his pals but one into consenting to the robbery, and toward the lone man who held out he felt a hot hate and fear; he had transferred his fear of the whites to Gus. He hated Gus because he knew that Gus was afraid, as even he was; and he feared Gus because he felt that Gus would consent and then he would be compelled to go through with the robbery.