Let $S\subset\mathbb{P}^g$ be a polarised smooth projective K3 surface of genus $g$ (and degree $d=2g-2$) over $\mathbb{C}$. Denote by $\phi: S\to G(3,g+1)$ the Gauss map, taking a point $s\in S$ to its tangent 2-plane $\mathbb{T}\_{S,s}\cong\mathbb{P}^2$ in $\mathbb{P}^g$. This is known to always be finite and birational, i.e. it is the normalisation onto its image.



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> **Q:** If $g$ is large enough and $S$ is general, is it true that $\phi$ is a closed embedding?
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If $g=3$ then the Gauss map $\phi$ of a general quartic K3 $S\subset\mathbb{P}^3$ is ramified along the Hessian curve in $\mathcal{O}\_S(8)$, on which it has degree 2, so in particular $\phi$ is not a closed embedding here.



Gauss map of general K3 surface
I think that a sufficient condition for the Gauss map to be a closed embedding is that each tangent plane $\mathbb{T}\_{S,s}$ intersects the surface $S$ only at the point $s$ and with the expected multiplicity $3$.


In turn, a sufficient condition to guarantee this is that the polarization is $3$-very ample: a paper of Knutsen "On kth-order embeddings of K3 surfaces and Enriques surfaces" characterizes completely the polarizations with this property, and in particular one gets that this happens for a general K3 as soon as $g\geq 7$.


