So today was my second time rolling over about 7-8 miles of serious gravel ...

I did a post-ride tire inspection today and found a few seemingly minor gashes and cuts on my tires.

Are these serious?

They look totally inconsequential to me. If they're just in the tread rubber and haven't cut the cords, ignore and ride. I have not found gluing cuts to be of any value.

Don't look serious from here.

Looks normal to me.

Not serious -- yet. But if you ride gravel with those tires a lot, eventually you'll get a cut or two that damages the cord. Then it will be more serious.

Do continue to inspect your tires regularly.

I found something that looks like a VERY small grain of rock which had worked about 1-2mm into the tread ... could that eventually cause a slow leak if it works its way through the tube?

The rock would have to work it's way through the tire's fabric caseing. That's harder to do than just embedding itself in the tread rubber. It can eventually happen however and, when it does, you'll definitely get a puncture.

Bicycle tire manufacturers usually offer models with varying degrees of puncture resistance. Essentially these consist of some kind of reinforced caseing so rocks can't penetrate it as easily. The most puncture resistant tires tend to be quite stiff so they ride and perform less well. The fastest performing tires tend to be very supple, puncture more easily and tend to not last as long.