Fill in the blanks: The two graphs are ___ or perpendicular to each other. If the product of the two gradients of two linear graphs is equal to , therefore the linear graphs are ___ to each other.

3 asks about the relationship between two graphs, providing "or perpendicular" and a hint "(parallel)". This suggests the blank should be filled with the other common relationship between lines.
The two graphs are $\boxed{\text{parallel}}$ or perpendicular to each other.

4 asks for the conclusion regarding the product of gradients for perpendicular lines.
Conclusion: If the product of the two gradients of two linear graphs is equal to $\boxed{-1}$, therefore the linear graphs are $\boxed{\text{perpendicular}}$ to each other.