It all depends on the child and the subject. If my first got less than As and Bs in high school math courses then that was a sign that something was not right. Getting a C in an art class was a reason for a high five. When my 2nd got Cs and the occasional B in her basic algebra class, that was a sign of celebration. If she failed to get less than a B in an art class, then that was a sign that something was not right. Same with my son. His strong subject is history. Right now in 10th grade, he is doing college level history and only getting Cs. Not because he doesn't understand the work or isn't studying or not turning in the work. His "problem" is that he is very weak in writing and high school history is all about paragraph answers to questions and essays. So his grades reflect that. Orally, he would be a straight A student in history.
Going from As and Bs in elementary school to Bs, Cs, and even Ds in middle school could be because now she has to learn. Something she might not have had to do in elementary school. Or because she is gifted, she never learned to study. Both are common "failings" of gifted kids. And middle school can be a big adjustment for some kids that has nothing to do with being gifted or average. Going from one teacher in one classroom with the same 20+ students to 5-6 different teachers in 5-6 different classes with 20+ different students in each class can be a tough adjustment to some kids.
I look more at the teachers comments than the grades themselves. Especially low grades in subjects that that child has no interest in or that isn't their strong suit. Poor grades in a subject that they are strong in and/or have a high interest in would be a different matter.