Gintama does indeed have a high viewership in Japan, just slipping into the top ten in its first week at #9 (
Video Research rankings). But actually Nana was the top new anime, at #5 (big surprise, since it shows at 11:30 pm Wednesdays, while Gintama shows at 7 pm Tuesdays). The only other new show in the top ten was Shoujo Changeum's Dream, at #10 (it shows at 7:30 pm Saturdays). The top ten is dominated by long-running kids' shows like Doraemon, etc.
Suzumiya Haruhi may be the best new show, and may have a lot of anime fans (like me, for example) very excited, but it's not really aimed at a mass audience, and shows at midnight Sundays. (Times are for the first broadcast of an episode. Shows appear on different stations at different times in different parts of Japan.)