So back from 2.5-2.9, gradebook 1 supported letter grading. 
What this essentially did when the property gradebook_enable_letter_grade=true was enabled was took the letter you entered, mapped against the course grade options and saved it as a numeric value. However it displayed in the UI as letter grade. A commit on
SAK-19964 looks like it inadvertently removed the option for setting letter grades, but adding that line back in gets it all working again. Gradebook2 also supported the letter grade style gradebook, probably inspired by this.
It seems like most of the code is in edu-services so it could be something that could get going again, though maybe in GB2 it wouldn't even need a separate type as an option, just if a user puts a letter in there A, B+, to convert it to the appropriate point value?