You can't just rewire a Saturn pad to DB15 as it uses data based connections and not just straight grounding for buttons like a DB15, you would need an external adapter to do the conversion.
There are multiple types of these adapters on AliExpress.
I believe you can use Sega Saturn pad but you need to remove communication chip on the PCB and then trace lines back to each input.
Of course when wiring DB15 wire you’d need to know what pin-out to use as not Supergun’s use the same controller standard (common being HAS Supergun pin-out)
Yeah but due to Neo Geo not having button 5 & 6 everyone chooses to map these definitely so just need to know where these are located on the pin-out when used with Supergun