I LOVE the Gap Shield Cover, yet it's only part of the solution for me. I reconfigured the way the Gap Shield Cover connects so it can be installed and removed somewhat quickly as I rarely use the bed for small particulates.
As
@DB Cooper stated first, get some heavy duty (firm - as the squishy stuff falls apart quickly), wide, weather stripping and apply around the opening of the tailgate enclosure.
My experience is the stock adhesive on the weather stripping will fail in about 2 to 3 months which I'm sure is commensurate with how often you open/close the tailgate. I ended up using Contact Cement to make it stick, going all the way around the frame of the opening the tailgate seats into. It's been almost a year now and no issues.
The one thing to note there will be a little resistance just before the tailgate latches as the weather stripping starts to compress between the opening and the tailgate, so it'll require a firm closing the first dozen or so times.
I know it's a long answer, but I'm a bit of a perfectionist ... yet my bed stays both dirt, dust and almost entirely dry, even through the car wash.
Hope this helps