There is nothing in canon that supports gnome saliva as being used as a recreational substance (I assume you mean as in like a drug).
A Billiwig is an Australian magical creature whose sting causes a euphoric feeling in the body, and then levitation. Billiwig venom is used in the candy Fizzing Whizbees and some witches and wizards purposefully try and get stung by a Billiwig in order to enjoy the giddy effects.
Those who have been stung by a Billywig suffer giddiness followed by levitation. Generations of young Australian witches and wizards have attempted to catch Billywigs and provoke them into stinging in order to enjoy these side effects, though too many stings may
cause the victim to hover uncontrollably for days on end, and where there is a severe allergic reaction, permanent floating may ensue. Dried Billywig stings are used in several potions and are believed to be a component in the popular sweet Fizzing Whizbees.
Fantastic Beasts and Where to Find Them - pages 4-5 - Scholastic
So while a Billiwig is not a gnome, it is an example of a creature used for recreational substance use.