foo.c: In function ‘gcd’:
foo.c:4:7: warning: statement with no effect [-Wunused-value]
    4 |     a >= b
      |     ~~^~~~
foo.c:4:11: error: expected ‘;’ before ‘int’
    4 |     a >= b
      |           ^
      |           ;
    5 |     int r = a % b;
      |     ~~~    
foo.c:6:9: error: ‘r’ undeclared (first use in this function)
    6 |     if (r == 0) {
      |         ^
foo.c:6:9: note: each undeclared identifier is reported only once for each function it appears in
foo.c:10:1: warning: control reaches end of non-void function [-Wreturn-type]
   10 | }
      | ^