"Babbling" is a precursor, to "talking."
The sound "gra"... as in "gramma" is difficult to 'say' at this stage of development.
"Talking" and enunciation.... is per a child's age and developmental age stage.... it does not all come at one time.
There are 'front of the mouth' sounds, 'back of the mouth' sounds, the tongue/throat/mouth/lips coordination too... in which talking is developed. Not all sounds... are 'mastered' at the same time, and will come with age development.
My son, from 19 months old until close to 3 years old, had Speech Therapy. And this is what I learned from the Speech Therapist.
My son, was simply delayed in speech, but advanced for his age in other developmental areas.
My daughter on the other hand, talked earlier. As girls often do. Boys being later.
But since you are concerned, just ask you Pediatrician.
As an aside, every State has an "Early Childhood Intervention" organization... that does FREE developmental assessments, including for speech, for children up until 3 years old. You do not have to be referred by a Doctor. I just called them myself. Just do a Google search for it, in your area.
Your son, is not yet 1 year old. My daughter at that age, was babbling like yours and saying simply sounds like "dada" and "mama." She was fine.
Again, if you are concerned, just ask your Pediatrician.
Not all kids, develop babbling or talking at the same rates....
For my kids when they were that age, instead of expecting them to say "Grandma".... I let them call her "Nana." Which is easier to say...
For example.
all the best,
Susan