Doomsday Supply is a deceptively powerful card. While it itself is far less glamorous than a Shotgun, LeMat Revolver, or Legendary Holster, the Doomsday Supply can often grab you those cards when needed. It can do that, and far far more.
One of my favorite combinations with DS is Disgenuine Currency Press. By having a currency press in your discard pile, DS can earn you 3 GR, and get a gadget on a dude like Prof. Eustace True, or Arnold Stewart to use with their ability. It's mid value is enough to pass the low difficulty gadgets, but might not be good enough for your high difficulty gadgets.
DS can also be used to put a goods onto booted dudes, and or dudes at locations you don't control, and even during a Shootout. Simply having a DS in play and a Shotgun in your discard pile can add threat against any low value dude in a shootout. DS is also good counter tech if your opponent uses Unprepared to remove a key weapons ability's and traits, by putting a fresh replacement on your dude and discarding the affected one. This can also help make sure a Force Field is functional. You could additionally use DS offensively to fire multiple guns off in a single shootout, providing you don't mind sparing the cost.
DS also combos extremely well with Pettigrew's Pawnshop, since you will get 1 return GR when DS played, and an additional 1 GR when the exchanged good enters play (if it's not a gadget), effectively reducing goods costs by 1 or 2.