Most gutter guard companies state the products work in heavy rain, some claiming they work in rain heavier than the heaviest rainfall ever recorded. However, customer reports show that some surface tension and fine mesh gutter guards don't perform as well during heavy rainfall—the water falls off them instead of into the gutters.
Before installing gutter guards, talk to neighbors who have gutter guards or ask several gutter guard installation companies for local references to confirm the effectiveness of different gutter guard types in your region.