Complementary angles sum up to 90 degrees. Given that angle JKL measures 34 degrees, we can calculate the measure of angle GHI (x) as follows:
\[ x + 34° = 90° \]
To find x, we subtract 34° from 90°:
\[ x = 90° - 34° = 56° \]
Therefore, the angle measure of ∠GHI is 56 degrees.