The first four are only a theoretical hydrolysis or oxidation step away from GHB. I doubt that it would be very easy in practice, as both ends of the molecules will react to varying degree in whatever procedure you use on them. Most of the substances seems to have become regulated from a theoretical standpoint only, as they are generally too expensive, too unstable and/or directly impractical to use as staring material for GHB/GBL.
The latter two are synonyms for 4-Hydroxyvaleric acid and gamma-valerolactone, respectively. More reading on those two substances can be found here: https://www.thevespiary.org/rhodium/Rhodium/chemistry/4-methyl-ghb.html (https://www.thevespiary.org/rhodium/Rhodium/chemistry/4-methyl-ghb.html)
Edit: I misread the above - the last one is gamma-butyrolactone, not gamma-valerolactone.
Post 439780 (https://www.thevespiary.org/talk/index.php?topic=9850.msg43978000#msg43978000)
(Aurelius: "Compilation on GHB/GBL", Methods Discourse)
why did I bother?