New here...here'* a pic of the frame on a 2015 GMC Sierra w/ 19K miles. One owner, every option available. Very nice truck otherwise Would this be a concern if you were considering a purchase? What options exist? Thank you in advance for your thoughts and expertise.

It it were me, I wouldn't worry about it.

I've definitely seen much worse, it appears to be only surface rust.
A bit of grinding/sanding and you are ready to undercoat it. Even those spay can rubberized undercoats would do well.
Of course there are professional shops that can undercoat as well.
My PA got treated with a Teflon based undercoat called Platinum Shield, good stuff.

Looks pretty typical for the age.

Five years and 19,000 miles and it'* that bad?

I agree with the others, grind then coat. It'* totally salvageable.

Still, I have to ask: Does it live in salt?

Oh ya that is salt corrosion.
It'* just common sense when buying a new vehicle the first thing that should be done is rustproofing,
Keep you vehicle clean in winter, power wash as much as possible under there.