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.

The statement asks for a relationship between two graphs, given the options "or perpendicular" and a hint "(parallel)". The blank should be filled with the other common relationship for lines.
The two graphs are $\boxed{\text{parallel}}$ or perpendicular to each other.

This conclusion describes the condition for two linear graphs to be perpendicular based on the product of their gradients.
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.