GC destroyer calculations are not meant for DAT that's why if you read beginning of the book Dr. Romano mentions to use calculator when doing the calculation. Other than that everything else is still the same. Have you watched chads videos? i would def watch those then start DAT destroyer and every topic you have difficulty in refer to GC destroyer and do that topic. Even after you finish with DAT destroyer, lets say you are taking practice test if you feel weak in one area on the test, again refers to GC destroyer. Doing that you're going to almost go over the GC destroyer or maybe all of it.
For Odyssey, yes i did use it. It is def tricky! but it prepares you for DAT destroyer O-chem. After I did the odyssey, G-chem destroyer was piece of cake. But it takes time to go over all the 24 chapters (40-60 problems). Again, depending on when you're taking your DAT, if you have time do what i did. But if you don't, go over DAT destroyer first then refer to odyssey on areas your weak on (just like G-chem, explained above).