I used an undercoat gun like you're looking at.
IIRC - it was 10 years ago - I think I transferred from the gallon can to the gun's can.
You might also be able to use a hose from the gallon to the gun.
Rick's stuff was better than anything else I tried, but getting the factory-looking thickness & texture seemed to require a lot of material and a couple applications.
Messy as hell, as all that stuff is; mask or cover everything around the area you're working on, even several feet away.