49ers mail call:Jason from San Francisco wants to know if I think Blaine Gabbert is definitely going to be the 49ers' backup quarterback this season.Bill Williamson: It appears certain that Gabbert will be Colin Kaepernick's backup. His contract of $2.1 million is guaranteed. The 49ers knew this when they traded for the former No. 10 pick,and they didn't add a quarterback in the draft although they did just sign Josh Johnson, who has a history with Jim Harbaugh.