There is a construction, you know that "Parents" is "Die Eltern", there is an ending "teil" which means "part" -- So you could say "Der Elternteil" to mean "the parent". I guess you could then say "Großelternteil" to mean a single grandparent, or a "part of the grandparents". I wouldn't know the gender and I couldn't imagine that it would be used in Deutsch very often...
It's definitely there, see: https://dict.leo.org/ende/index_de.html#/search=Gro%C3%9Felternteil
https://de.wikipedia.org/wiki/Gro%C3%9Feltern (used several times in this page).
Dulingo says Großeltern is NEUTER, therefore DAS Großeltern. But "DEINE Großeletern" assumes it is FEMININE. I'd say, once it's plural, I should stick with the feminine form but again when you hover your mouse in Großeltern it clearly says NEUTER. Platform error? My mistake? I'm confused