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Looks like it was wet sanded, then polished. I wouldn't recommend wet sanding unless you know what you're doing.
Not mine but I've cleaned that sort of thing up many times. I've got several types of compounds to fix this if the paint isn't removed.
If the transfer of paint was mostly TO the GS, a good rubbing compound, either fine machine or courser hand, will take most of it off easily. Then you just need to put a little polish/wax on it to finish it up and blend it in.
I look forward to hearing the method the poster used.
Who's splash guards did you use? The lighting is such and your car color makes it hard to tell if they color matched or black/gray.
Got them off of amazon as well, they're unpainted (black). I thought they'd be a rubbery material, but they ended up being a hard plastic, which made it difficult to install in the cold.