Taking the DAT will depend on the school you are applying to... Some schools want to have the DAT taken by October some want it by the end of NOvember. SO, it depends. Check with the schools you want to get into. I know for Case WEstern Reserve, you can take it in DEcember but will most likely be on the alternate list.
As far as GPA, I understand that they will look at every course taken from any college attended.