GHB was found in low concentrations in a few plants, also in Nymphaea lotus:
I hope the pdf-link works
name of the study from 2007: "CONSTITUENTS OF NYMPHAEA LOTUS LINN."
https://www.google.at/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CDsQFjAB&url=http%3A%2F%2Fwww.unilag.edu.ng%2Fopendoc.php%3Fsno%3D17620%26doctype%3Dpdf%26docname%3D%24&ei=mjwzUcyKOeuQ4gSFoIDYDw&usg=AFQjCNHJm-0l9rfI7x6s2nuklNt04o5cNA&bvm=bv.43148975,d.bGE
GHB (gamma hydroxy butyric acid) is mentioned in this study with the synonym "4-hydroxy butanoic acid"
Nymphaea lotus often has its leaves under water (in contrast to N. caerulea and Nelumbo sp.) and is available as plant in aquaristic stores
GHB was also found in Quercus suber, fermented products like aged meat, tobacco and wine and Hippocrepis comosa, GHB-glucosides in Anoectochilus and Goodyera species
The GHB-precursor GBL (metabolically converted to GHB) was also found in plants like Rhododendron ponticum and Zanthoxylum piperitum (suspension culture).
The GHB-analog Funebrin was found in Rosita de Cacao (Quararibea funebris)- flowers
Of course this is only for scientific information, anyway an extraction would be inefficient, regarding the low amounts in these plants.