Pharmacology - Has phenibut usage blocked the GBL effect?

[m24]

Swim has been using phenibut two or three times a week, not on consecutive days (one 4/5 gram dose each day of usage) for around a year and a half and built up a good tolerance to it but still feels a decent effect from it at around 4/5 grams. Swim has lowered his usage to once a week at around 1/2 grams lately to try and reduce tolerance.

Swim recently aquired some GBL alloy cleaning liquid and tried some varying dosages over the last few days, 1ml on an empty stomach with 300ml grapefruit juice didnt make swim feel much at all, then the next day 1.5ml, light beer like buzz, next day 2ml, slightly more beer like buzz, then 2.6ml today resulting in a drunkenish feeling and noding off for 15/30mins. Swim tried dosing another 1ml about 50 mins later and simply felt slightly drunk/sleepy again. All this with no feeling of euphoria or body buzz like phenibut has given swim and still gives swim even though the best days of phenibut are clearly over.

Is it possible the phenibut usage has somehow worn down some kind of brain receptor in swim and if so, why would the phenibut still work after all this time (albeit weak) and the GBL not? Does GBL take a good few goes before someone starts to feel the euphoria and buzz?