Got a package this week from the Distinguished Gentleman from Alabama, GusRobin

Very generous packs of both the Gunpowder Seasoning and Star Market Meat Rub (aka Red Gunpowder). Many thanks, Gus!
It didn't take me long to tear into the GP to see if I was close. I think I was on the right track, but missed the mark by a good bit. At about the same time that I was getting my first taste of real GP, Chris (Tenpoint5) was sending me pics and notes on his testing. After both the two way tasting and comparing notes I planned version 2. Here we go.
The things I need to change were get more grill flavor, back off the MSG a tad, back off the onion, garlic and black pepper, and eliminate the ground Jalapeno powder. It would be pretty simple, except for the grill flavor, which I can't source, so I improvised.
To get more grill flavor I modified my "grilled salt" by using Morton Canning & Pickling salt (finer grain, more surface area), changing the veg oil to pure soybean oil and doubling the amount of oil, and increasing the temp to 600 - 650 °F with indirect heat on my gas grill and grilling for 35 min's. The canola component in the first version gives a slightly acrid note in the grilled salt that I wanted to avoid. If ever you decide to try this, do it outside. Just trust me on that one. The new salt formulation is as follows:
300 g pure salt
18 g soybean oil (Crisco Pure Vegatable oil)
Using a KA mixer I mixed the oil with the salt for ~ 5 min on speed 2, spread it in a SS roasting pan, put it in a preheated 650 °F gas grill. The salt was well mixed every 10 min's, grilled for a total of 35 min's, then the pan was removed from the heat, covered tightly with aluminum foil and allowed to cool for ~ 1 hr. This gives a finished grilled salt with at least 2.0% weight gain from charred veg oil.
After a bunch of back and forth tasting, sniffing, and adjusting, here is the final formulation for GP clone version 2:
Grilled salt (v2) 300 grams
MSG (Accent) 50 grams
Onion, granulated 15 grams
Garlic, granulated 15 grams
Black pepper, ground 15 grams
Savory, ground 6 grams
Smoke Flavor (Leggs 127) 6 grams
Soybean oil 3.2 grams
Combine everything except the soybean oil in a KA stand mixer fitted with the paddle and mix on speed 1 for 2 min's, then on speed 2 for 5 mins. Add the vegetable oil in droplets across the top and then continued mixing on speed 2 for another 5 min's. Transfer finished blend to an airtight container.
Flavor on this version is very close, but to me still lacks in the grill flavor notes, which isn't surprising given that I am improvising for grill flavor that I cannot buy.
Visually the clone is noticeably coarser than the original, even with the salt granules. I could run the clone through a blade grinder in small batches, but I doubt it would change much of anything other than the appearance, which I don't believe is all that critical.
Here is a comparson of the original from GR (left) with the new clone (right).

Samples will go out to the sensory evaluation panel tomorrow (GusRobin, Tenpoint5, and CRG) and hopefully there will be some feeback next week.
Honestly, I think this is probably about as close as someone at home can come up with. A major key to GP's flavor is the grill flavor, and that just isn't available (yet) in the retail market.