Is grampa correct spelling or grandpa?

The possessive grandma's means pertaining to ONE grandmother.

(Note : used as a name, it would be capitalized Grandma's, referring to your grandmother. If not a name, it is not, as with someone else's grandmother (e.g. his grandma's recipe).

The plural possessive grandmas' is less used, and means of MORE THAN ONE grandmother (e.g. both contestants used their grandmas' recipes).