refactoring: changed to Euclidean modulo algorithm

src/ggT/ggT.c

@@ -2,12 +2,10 @@
 
 
 int ggT(int a, int b){
-    while(a != b){
-        if(a < b){
-            b -= a;
-        } else {
-            a -= b;
-        }
+    while (b != 0) {
+        int temp = b;
+        b = a % b;
+        a = temp;
     }
     return a;
 }