Gibson finished sixth, seven seconds behind Alexander.
His efforts still moved him up into fourth overall in the series over teenager Marcus Armstrong, who did not contest the round, with Callum Quin coming up to fifth overall.
With two rounds left, Gibson's 567 points is still well behind the top three, with Blewett sitting on 680 while the title will be fought out between McNee (732) and Alexander (750) after the Cantabrian moved ahead of the Wellingtonian over the two South Island rounds.
The next round is at Manfeild, February 14-15, before the series finishes at Hampton Downs, February 28-March 1.