Given a parent genotype of Bb, what are the possible gametes that they can produce?

Assuming both parents have genotype Bb, this would lead to the Punnett square (in this case a monohybrid cross) as shown in the image below. We have four outcomes, three of which are unique. Those offspring genotypes are: BB, Bb, and bb.

An offspring with B or b on its own is not possible because both parents contribute genes.