Primer is not made to cover, it's made to seal and prime.
Since you are using Gardz, it will serve as a primer.
Now, if you skim coat with drywall mud over the Gardz, you are going to have to seal the skimmed spots. You can just spot prime over those spots though, and you can use the Gardz to do that so you don't have to buy a white primer.
Most quality acrylic finish coats are self priming. Read your label thoroughly and it will tell you if it's self priming or not.
Good luck,
Phil