We always book through the casino and all they give us is a five or ten percent discount - pitiful.
The only free ones are annual tier and what is earned from play on a sailing. I've inquired several times and they give a canned response saying it is based on several things, blah, blah, blah.
DH gambled once on a Carnival cruise a few years ago and he gets daily offers for complimentary cruises from them (but no thanks).