The Great Old Ones is a collective name for the beings which existed before the current Doctor Who multiverse. These include Elder Gods, Guardians of Time, Eternals and many more. They have godlike powers and exist in multiple dimensions at once.
The Grace worked to hold the universe together and occasionally stepped into reality to prevent it from falling apart.
There are various accounts of The Doctor having met with others such as Cthulhu, Yog-Sothoth, and others.