Step-by-step explanation
Since we have the triangle, and the measure m
We have two complementary angles inside the angle DBF,
  m
Substituting terms:
 x - 12 + x = 90
Adding like terms:
 2x - 12 = 90
Adding +12 to both sides:
 2x = 90 + 12
Adding numbers:
 2x = 102
Dividing both sides by 2:
 x = 102 / 2
Simplifying:
 x = 51
Finally, by the Vertical Angles Theorem, we can affirm that the angles GBC and DBE are both congruent.
Plugging in x=51 into the given equation:
 x-12
 
 51-12=39
In conclusion, the measure of the angle GBC is of 39 degrees