I believe it’s based on how many possible words are left. I still had 5 words left after my fifth try. So it was definitely appropriate.

Gradle's letter grades are bogus. They're based solely on what percentage of remaining possibilities were eliminated during that guess. To get an A- on your first guess, you have to leave only 21 possible solutions, which is mostly luck. The guess that led to my defeat today was scored a B-, where the words I should have played instead would have scored worse in Gradle.