Complementary angles are two angles whose measures add up to 90 degrees. Given that angle JKL measures 34 degrees and angle GHI is represented by x degrees, we can set up the following equation:
\[ 34 + x = 90 \]
To find the measure of angle GHI (x), we subtract 34 from both sides:
\[ x = 90 - 34 \]
\[ x = 56 \]
Therefore, the angle measure of angle GHI is 56 degrees.