Each weight gradient is the upstream gradient times the input feeding that weight. The branch with zero upstream gradient contributes exact zeroes.
First hidden-unit gradients
For the first hidden unit, dL/dz1=-2. Multiplying by x1 and x2 gives dL/dw11=-2 and dL/dw12=-4.
dw11dL=−2⋅1=−2,dw12dL=−2⋅2=−4
Biases and the blocked unit
The first bias gradient is dL/db1=-2. Since dL/dz2=0, the second hidden unit has zero weight and bias gradients.
db1dL=−2,dw21dL=dw22dL=db2dL=0
Summary
The parameter-gradient register is exact: dw11=-2, dw12=-4, db1=-2, and the dead branch entries are all 0.
dw11=−2,dw12=−4,db1=−2