"He said just to put a gallon of gasoline in my tank, and that it would do the same thing. "
Not hardly. Back in the old days we used to use a gallon of unleaded in 100 gallons of diesel when it was really cold. -30 or lower. It wouldn't let the fuel gell up. Beyond that, the Howes claims to be a lubricant and I don't think anyone would say that gasoline is. Howes says their stuff is approved for use in all diesels. I don't think anyone actually "approves" of gasoline as a diesel fuel additive except in extreme conditions, and then only in very low concentrations. Howes claims overuse of their products won't harm the engines fuel system. Not so with gas. Howes has been around a long time and is used by lots of folks with no apparent ill-effects. Also not so with gasoline.
Another old timer deal was you could actually fuel your diesel on gasoline in a pinch if you added 1 quart of 30 wt. non-detergent oil for every gallon of gasoline you put in it --- which should be an indicator of the amount of lubrication gasoline does not afford the fuel system. And finally, these old timey fixes were employed back when even heavy duty line haul truck diesels only made it 150,000 miles or maybe 200,000 or so without needing a complete overhaul. Now, they routinely go 850,000 or even 1 million miles with nothing but routine maintenance or maybe a set of injectors at 700,000 miles or so. Tolerances back then were really loose compared to today.
I wouldn't use the gasoline unless I was really pressed, as in bitterly cold and its better to keep the engine running and stay alive rather than let it gel up and you freeze up along with it. My $. 02.