Yeah I never had any success finding part 25 style climb gradient info for part 23 airplanes either. Maybe nobody has gone to the trouble of doing a full blown performance engineering study and validating the derived data with a flight test program? I've had to estimate it myself using the ROC data from the POH to determine as best I could whether I could meet climb gradient requirements for certain SIDs and missed approaches. A whiz wheel and/or calculator used to make conservative estimations is better than guessing I suppose.
Anyway I hope that wasn't too much more than you were asking for and happy flying!
westhawk