Damage Frost Death Knight wants Def gems?

hey, first post.

quick question, why does it say that my frost dk needs def gems in my damage gear?
i cant even select tanking so why does it suggest def gems?

If you post a snapshot we can take a closer look, instructions on how to do that here:

Without any further information: my guess is that you have a Blood setup with a higher priority for Best in Bags than your Frost setup.