Solid line = floor (spellings that unambiguously belong to the group). Shaded band up to the dashed edge = ceiling (the most it could reach if every ambiguous spelling counted here).
Gwendolyn was more common in the past and has since settled at a lower, steady level. All-time it's been given to 124,718 U.S. babies, the 783rd most common name by sound today, peaking at 3,531 in 1953, with 848 in 2025.
Gwendolyn has no common alternate spellings by sound.