Well, considering the fact that on that particular day, a typhoon hit Japan and just about everyone was stuck at home, I'm not surprised at all...
Anyway, the fact is Gundam was NEVER in a weekday primetime slot unlike shows like DBZ and Eva. It's almost always either in a weekday slot before primetime (5:00, 5:30, etc), or in the case of SEED, in a weekend slot which generally does not score as high a rating. (and Gundam X had 13 episodes on a 6:00 AM Sat morning slot which is one of the worse possible).
So it's not surprising that Gundam's don't get as high a rating as those other shows. And it doesn't really need it either since the main source of income for Gundam is merchandising.