Reverence is the response of humility, humility in the face of a natural world whose depth and breath far exceeds our capacity to know, now and perhaps forever.
Some folks cannot consider the natural world without imagining a humanlike creator. This is apparently a completely natural response. Piaget showed us that anthropomorphism and artificialism are the default explanations of the child. They have certainly been the way most humans have explained the world since time immemorial. And so it has been common to direct one's reverence away from the creation to an anthropomorphic creator -- in effect, reverencing a projection of ourselves.
The religious naturalist considers this a mistake. It means replacing humility in the face of the entirety with humility in the face of a part. Granted, there is ample mystery in a human self to excite awe and reverence, but a human self (or its projection as a divine person) is a fragment of the whole of creation. Whatever is the referent of the religious naturalist's reverence, it is inexpressible as some ensemble of human attributes.
The religious naturalist can sing a Te Deum as robustly as anyone, and can be awed by the ceiling of the Sistene Chapel, but the ultimate expression of her humility is expressed in a reverent silence.
And gratitude? Does gratitude require a giver? Yes, I suppose it does. But the giver need not be known, and certainly need not be a person, divine or otherwise. Life is a gift. Beauty is a gift. Love is a gift. The universe as we find it, in all of its majesty and mystery, is the religious naturalist's gift, and her response is gratitude.
'Tis the gift to be simple,
'tis the gift to be free,
'tis the gift to come down
where we ought to be,
and when we find ourselves in the place just right,
'twill be in the valley of love and delight.
When true simplicity is gained
to bow and to bend we shan't be ashamed,
to turn, turn, will be our delight
till by turning, turning we come round right.