In recent years I've stopped doing it gutless, in spite of the extra work and mess involved, and I haven't had a tough one since.
Why would gutting or not affect toughness? Are you using gutless to mean boneless?
You need to separate the meat from the gut to promote cooling and to prevent the meat from picking up a bad taste as the stomach juices start to break down. Gutting removes the gut from the carcass. Boning removes the meat from the carcass. Either way, the meat and gut get separated.
Gutting has no affect on tenderness. The problem is that meat removed from the skeleton too soon will contract as it cools and that's what causes toughness.
