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This is a good question. As I understand it, the only material parameters a splat can recognize are color and transparency. Therefore the first of your two options would be the correct one.

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You can use spherical harmonics to encode a few coefficients in addition to the base RGB for each splat such that the rendertime view direction can be used to compute an output RGB. A "reflection" in 3DGS isn't a light ray being traced off the surface, but instead a way of saying "when viewed from this angle, the splat may take an object's base color, while from that angle, the splat may be white because the input image had glare"

This ends up being very effective with interpolation between known viewpoints, and hit-or-miss extrapolation beyond known viewpoints.

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