It's also not how plasma works. Unless speculative matters include physical laws. Given this is Anime...I guess that's entirely possible. You just aren't going to contain a plasma on the outside of a single magnetized slug in a meaningful way.
The only particularly weaponizable way of shooting plasma at something is to somehow get it compressed inside of some kind of containment so it can't radiate it's heat out very well. But you're talking a lot of energy to do that.
It makes far more sense to use available energy accelerate a heavy slug than to heat up something up until it ionizes.
When you start talking railgun speeds...that slug is either going to blow a hole right through or, more likely, melt/vaporize/ionize itself and whatever it's hitting.
But Like I said...that doesn't really matter....it's anime they can do whatever they want to physics
Still, similarly to Killer Robot, I think we might be hitting some translation snags. I wouldn't know: what's the difference between sabot and cartridge in Japanese?
I mean...Gold makes sense to use as a component of railgun ammunition because of its conductivity. It doesn't make too much sense as the actual penetrator...it's not very hard. DU and tungsten are both better options.
I suspect the gold is actually used as a sabot. It could very well be totally ionized in the firing process by the way, if you've seen on go off - that flash is plasma from the projectile/sabot being ablated.