First, let me congratulate you on spelling both the company name and product name correctly. I think you are the first person in the history of this forum to do so.
Gardz is a great product to use for priming drywall. I'm not sure why it doesn't get more use this way. It could just be tradition - it's not a standard white primer. However if people really want what they say they want - i.e. sealing of porous surfaces - Gardz is the way to go. For how hard some people scream about how important it is to prime, you'd think they'd want to use something that really seals. And frankly a lot of drywall primers don't seal very well.
Regarding the concern about the need for pigment coverage that Gymschu mentioned, unless you're tinting the primer a very different color to match a dark paint, I don't totall agree. A good quality paint should be able to cover that opaquely in 2 coats, IMO. But, it's possible it's an issue sometimes.
I personally usually don't use a primer over bare drywall, with the modern quality paints that are self-priming over new drywall. One reason is the primers don't solve the problems as well as they advertise, or in other words "paint and primer in one" so-to-speak paints work basically as well as a separate primer coat in this situation. If I need a "real" primer, I usually use Gardz.
I think most painters should read this.
https://jackpauhl.wordpress.com/tag/gardz/
One thing to keep in mind is that a coat of paint over Gardz is going to take longer to dry than over bare drywall. This tells you it's working, i.e. the paint has to dry exclusively out to open air - the water can't soak into the drywall and dry out faster that way.
Edit: I removed the obviously wrong line where I said new drywall is all white.