It’s my understanding “gentlemen’s C” refers to a past era where students would do unsatisfactory in the course, such as not understanding the concepts or simply not doing the course work, such that they are deserving of a failing grade. However, rather than a failing grade, they would be awarded a C, in some cases due to name or social standing.
In more modern times, it is extremely rare for students to do unsatisfactory in the course, such that they are deserving of failing grade at schools like Harvard. Some students master the material to greater degrees than others and are more/less successful in the course than others, but the class is full of high achievers who put the effort in to do the course work and have adequate understanding of the concepts. Extremely few students are deserving of a failing grade, which makes it a matter of speculation what grades those few students would receive . For example, what would happen if Malia Obama stopped doing coursework for her classes and did not sufficiently understand class concepts, as reflected by unacceptably poor exam grades. Would the professors give her a A/B/C because of her name/status in spite of being deserving of failing? Some colleges do not report failing grades on transcripts. For example, Brown does not list grades below C on transcripts, which could be considered a form of a modern gentlemen’s C.
I expect “gentlemen’s A-” refers to general grade inflation, with the average grade for many courses being A-/A. I’d expect the students who receive A- grades generally do the coursework, understand the concepts well, and are absolutely not deserving of failing grade – very different from “gentlemen’s C”. I expect Harvard and similar colleges do generally have a large portion of underclassmen math/science courses with average grades of B+ or B, but I’d expect upperclassmen major-specific courses or courses that are filtered for top performers (math 55) usually have average grades of A-/A. In general, when a larger portion of students sufficiently understand or master the material, the portion of students receiving A grades goes up. The portion of A grades also goes up when grades are primarily based on subjective metrics, such as papers; rather than objective metrics, such as quantitative, numerical problems.
Regarding athletes, athletes do usually average lower college grades than non-athletes. For example, a list for Berkeley is at https://asc.berkeley.edu/sites/default/files/aps_2018_d_fall.pdf . It looks like the average athlete GPA prior to COVID was near 3.0. In contrast, the average grades for non-athletes was probably near 3.4 during this period.
Stanford seems have a larger degree of GPA variation among teams than Berkeley. Several of the non-revenue generating sports average similar or higher GPA than non-athletes. For example, Women’s indoor volleyball has averaged a ~3.8 GPA in recent years, with several team members above 4.0. However, the revenue generating sports average much lower GPAs. I believe football often averages near 3.0 college GPAs, which pulls down the overall average across all athletes to ~3.4. An old article from 2008 mentions Stanford football matriculating students averaged a 3.68 HS GPA and 1176 SAT (other Pac 10 schools all were <= 3.15 HS GPA and <= 990 SAT). At the time, the overall class averages listed in CDS were 87% >3.75 HS GPA and ~1430 SAT.