This term was used in this way, for the first time: “The oyster is a blob of glup, but a woman is a woman.” ― James Thurber, The 13 Clocks . I also saw some articles using this phrase, but I just don't get it.
'Glup' seems to be a (possibly) Thurber-invented term meaning roughly 'slimy, jelly-like material', similar to guck.
I believe the remark is intended to be humorous (using bathos), contrasting a slimy food, offering a small amount of transitory pleasure to the small number of people who like oysters, with the much more fulfilling possibilities offered by interaction with a woman. Presumably the remark was written to be understood from a traditional sexist male viewpoint. To be contasted with the similar rather misogynistic remark by Groucho Marx, borrowed from Rudyard Kipling: 'a woman is just a woman, but a good cigar is a smoke'.