Marriage? A personal commitment? yes.
A contract with a religious body or government entity? Dagny and John? Have they learned nothing?

What would Dagny want from John if they were formally committed?
A fair exchange, value for value.

No, John & Dagny won't go to a priest. It would likely be Judge Narragansett, who married Ragnar and Kay. And, again, Ayn Rand loved symbolism, so it's fun to reflect not on the Lord of the Rings or the Ring of the Nibelung but on Dagny's ring!

No, he would not have offered her a ring at all but a chance to work with him on his next invention as a true co-inventor. Possibly he had found a way to marry Tesla's work with his own ... and move physics way past where it currently resides. The "ring" would have been the opportunity to take part in that dream.

I think the ring itself might well have been made
of Rearden Metal. As for the stone, I don't know.

John Galt would have given her a ring he MADE using his brain, whether it be coal compressed to a diamond, a ruby he dug out of the ground or just a simple band of gold with a classy statement of his commitment to her.

That's easy.

Rearden metal band, and the main 4-carat stone computer laser-cut from the core of the rarest asteroid, chosen for its one-of-a-kind lattice molecular arrangement that produces a sparkle 20 times brighter and more colourful than the purest and most flawless diamond, and includes phosphorus atoms which cause the stone to sparkle in the dark.

Inlaid in the Rearden Metal band either side of the stone are smaller cuts of the same stone, arranged in the pattern of a dollar sign.

Hi Dr. Edward Huggins and happy holidays to you and your family!
Yes and no.

Yes the appreciation of invention
(man-made diamond) makes perfect sense.

But , No I don't agree with the premise that Dagny
would marry John Galt. Dagny as a totally self sufficient ,intelligent reasoner would know that she is a producerizer. A woman who is chemically attracted to the inventor , the creator , the problem solver.

Like a womanizer bachelor such as Al Pacino who covets beautiful women His little black book reads like a Hollywood who’s who of actresses: Tuesday Weld, Ellen Barkin, Debra Winger, Diane Keaton, Jill Clayburgh, Beverly D’Angelo. Even Winona Ryder and Rose McGowan have been rumored to be attached to a much older, and more experienced, Pacino.
While he’s never tied the knot with any of his paramours, he has fathered three children: a daughter by acting teacher Jan Tarrant and twins with D’Angelo, whose split from Pacino was famously acrimonious.

She loved with all her heart and womanhood Francisco when their stars aligned
for them. His work ethic ,brains ,energy ...... When Francisco was Educated and become part of the trinity. Their stars were at odds.
Her attraction to Hank Reardon was again overwhelming. On the train, the success of the new bridge and track built with Reardon Metal on the inaugural run . She was ecstatic, the happiness ,the joy. Wyatt Ellis's lodge could have been a cheap motel and they were still going to connect physically. Hank like Francisco was the total package.
In a sequel to AS I imagine the rebuilding would occur as the government dissolved and was reformulated guided by Judge Narragansett (spelling edit). Certainly Galt and Dagny would have physically and emotionally connected as it fit the moment. Dagny would have a railroad to run. Galt an Automobile Company and engine manufacturer As she knows the next innovator ,the next bridge builder, the next tunnel engineer. The next day of life provides the potential for a river of joy, ecstasy ,fulfillment and excitement . Like capitalism, the build up of markets and their collapse from the next great innovation. Dagny would follow her life ambitions and the rise and fall of lovers.
Regards,
DOB