The measure of arc GC is 90°
Solution:
Given data:
m∠CHD = 90°
Sum of the adjacent angles in a straight line = 180°
⇒ m∠GHC + m∠CHD = 180°
⇒ m∠GHC + 90° = 180°
Subtract 90° from both sides, we get
⇒ m∠GHC = 90°
The angle measure of the central angle is equal to the measure of the intercepted arc.

90° = m(ar GC)
Switch the sides.
m(ar GC) = 90°
The measure of arc GC is 90°.