
Koufax and Gibson pitched against the likes of Mays, McCovey, Aaron, Clemente, Banks, Robinson, Cepeda. BTW, you might want to remind your friend that Gibson is African-American.
Gibson and Koufax could probably both go out to the mound today, at their age and be respectable. I've heard that Koufax will still throw some bp in spring training and hit the mid80s.
The size of ballparks has not changed much, there's always a mix of "hitter" and "pitcher" parks. Plus Gibson and Koufax pitched when there were only 8 and then 10 teams in the league. There is the argument that players may be bigger and stronger (at least for the last few steroidal years

While Clemens immediately comes to mind, can't think of too many current pitchers that could hold a candle to Gibson or Koufax.