These are much like a QPRT. The main difference is that a GRAT or GRUT lets you transfer an income-producing asset (stock, real estate, business) to a trust for a set number of years, removing it from your estate — and still receive the income. (If the income is a set amount, the trust is called a GRAT. If the income fluctuates, it’s called a GRUT.)
When the trust ends, the asset will go to the beneficiaries of the trust. Since they will not receive it until then, the value of the gift is reduced. If you die before the trust ends, some or all of the asset may be in your estate.