∠R is congruent to ∠G
The reason: the right angle's orientation gives a reference for which angle, if the triangle is congruent, is congruent to which angle.
OR
You can follow the letters. If the triangle is labeled with the letters or numbers, it is 90% of the time going to be in order of the alphabet or the numerical sequence.
ΔGHI ≅ ΔRST
Notice how those letters are in order? The ∠G is congruent to ∠R.
Hope I could help you out! God bless!