$ id
uid=1005(buildd) gid=106(sbuild) groups=106(sbuild),1005(buildd)
$ mkdir foo
$ echo a > foo/bar
$ chmod 750 foo
$ sudo chown root.buildd foo
$ cat foo/bar
a
$ cat > test.c
#include <stdio.h>
#include <unistd.h>
int main(void) {
printf("%d\n", access("foo/bar", F_OK));
perror("access");
return 0;
}
$ make test
$ ./test
-1
access: Permission denied

Apparently access() restricts itself to the real gid, without including
supplementary group IDs.