According to this strip, it looks like that Grandma Duck is actually Donald's mother.
Worth noting is that in the continuity of this strip, Elvira is both HDL's grandmother and Donald's! He calls her "Grandma" too! I mean, you wouldn't call your mother that, right?
Related to the topic, I thought I'd post this one too, from Rosa's WHADALOTTAJARGON. At least Rosa's version of Elvira clearly doesn't think it is necessary too be specific enough to include the "great-" in "great-grandmother":
I think it's typical for English-speakers *not* to use the "Great-" part of a familial title in direct address. A great-grandmother would usually be called "Grandma" (or "Nona" or whatever--when there are several women who could qualify as "Grandma," families sometimes come up with different "grandmother" nicknames for them). We called our great-aunt "Aunt Minnie," not "Great-aunt Minnie." While people might introduce or point out someone and say, "She's my great-aunt," I don't know anyone who uses the form of address "Great-aunt Jane." Everyone I know just says "Aunt Jane" as the form of address. Has anyone heard or read otherwise? I'm trying to remember what form of address is used for great-aunts or great-grandmothers in, say, Jane Austen....
I'm interested in knowing what Barks means to this "grandpa" from The Hard Loser. In Finnish he has been translated "naapurin pappa" (the old-timer who living next door) which suggests that the old-timer is not a relative but a random neighbor with a horse. But is this even the original purpose of Barks, because in English the same text doesn't suggest that he is Donald's neighbor?