Question 41874
Two triangles are congruent if a line angle line are congruent.  or angle line angle are congruent.  So you need two lines and an angle that connects them to be congruent for the triangle to be congruent.  When a line is perpendicular then each of the bisected parts are going to be congruent, there is one leg of the triangle.  GF is going to be congruent because it is the same line for both triangles.  The angle is going to be congruent becuase it is formed from the perpendicular lines.  Hopefully this made this as clear as mud (haha).