If it's not needed, then you can roll it down to other relatives - the definition of relatives is fairly expansive and certainly includes great-grandkids if you set it up with DGS as a beneficiary. (It's not quite "everyone" as NCC1701 states above, but not sure what happens if you transfer it from DGS to, say, your next door neighbor. Probably not an issue in your case anyway.)
You can also take distributions which are non-qualified (i.e. not used for educational expenses) and the beneficiary owes ordinary income taxes plus a 10% penalty (Yes, @NCC1701 that's the right percentage) on the portion of the withdrawal that is attributable to earnings. You can avoid the 10% penalty to the extent that there are scholarships that the beneficiary received.
What I did was take a NQ distribution for my oldest son during the fall of his senior year. The 10% penalty was avoided because he had earned scholarships his freshman year. He also paid no income taxes because as a FT college student that year he had zero income.
And yes, you can use it for grad school, as long as it is for qualified educational expenses as outlined in Pub 970 (essentially tuition, books, fees, equipment, room and board if at least half time, and computer stuff).
Finally, I'll add that the SECURE Act added the payment of up to $10K of student loans as a qualified educational expense, so you could use 529 money to pay off the $7500 student loans of DGS if you wanted. You'll have to assess the probability of student loan forgiveness vs. paying the interest to decide if you want to use this provision. There is also a student loan interest and fees deduction which may apply for DGS if you decide to keep the loan.
Oh, one other thing. Some states (mine included) have a state income tax deduction for 529 contributions and no requirement on how long the money needs to stay in the account. So what you can do is "529 rinse" things by opening a 529, contributing, say, $6K into the 529, taking the money out a day later and pay off $6K of DGS's student loan or fall semester of grad school, and then deduct $6K on your state income tax return for that year. (Not all states have income tax deductions, and the limit may be higher or lower than $6K, and some states prevent 529 rinsing - do your own due diligence on this part, but the principle exists regardless.)
ETA: One more thing. DGS probably will qualify for the Lifetime Learning Credit. Just be sure not to double dip - you can't take the LLC for expenses paid for by the 529.
"At times the world can seem an unfriendly and sinister place, but believe us when we say there is much more good in it than bad. All you have to do is look hard enough, and what might seem to be a series of unfortunate events, may in fact be the first steps of a journey." Violet Baudelaire.