How to calculate individual offensive and defensive ratings
www.basketball-reference.com
This is how it's calculated. One of the big weaknesses of the measures (IMO, not much of a stats guy) is that it doesn't always account for bad vs good team. A player on Georgetown is going to have a worse defensive rating just because the entire team was poorly coached and it made even average defenders look bad.
Gonzaga and UConn have similar enough SOS and are well-coached teams, so I think comparing the stats is more likely to have good results.
@auror and others would probably know more than me. I'm making a lot of assumptions here.