Hi there!
We know:
∠AGB ≅ ∠DGC because they are vertical angles. They both are 90°.
∠AGE ≅ FGC because they are vertical angles, equal 30°.
∠BGF ≅ ∠DGE are vertical angles, both equal x.
All angles sum up to 360°, so:
360° = 90° + 90° + 30° + 30° + x + x
Simplify:
360° = 240° + 2x
Subtract:
120° = 2x
x = 60°