It doesn’t happen often, but it _is_ possible to stop being allergic to something, and food-based allergies like eggs and milk are apparently the most common to outgrow - it’s possible Gobo could have just outgrown his allergy to whatever ingredient in radish bars he was allergic to! Red does ask him if he’s _still_ allergic to radish bars in the episode that establishes his allergy, after all.
Every time i watch this video, whenever they get asked “does anyone in fraggle rock have smelly breath or smelly feet” i imagine gobo hesitating before red bluntly going “…large Marvin. large marvin has smelly feet.”
If there was any kid of the 1980s grew up watching FR on HBO just like I did Then this should also discover kids that were not born in that era down at FRaggle Rock