According to Harry Potter Wiki Xenophilius Lovegood tells his daughter
"Gnome saliva is enormously beneficial! Luna, my love, if you should feel any burgeoning talent today -- perhaps an urge to sing opera or to declaim in Mermish -- do not repress it! You have have been gifted by the Gernumblies!"
Does this mean garden gnome saliva is used for recreational purposes in the Wizarding World?
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).
HOWEVER!
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.
Wizards almost certainly do not use gnome saliva to experience its benefits, because those benefits are likely untrue and only believed in by the Lovegoods. While there are magical creatures that can produce substances that can be used by wizards, like the melancholy-inducing treacle produced by the Glumbumble and Ashwinder eggs increasing mental agility, as described in “Fantastic Beasts and Where to Find Them” there are no special properties of gnome saliva mentioned.
“The gnome is a common garden pest found throughout northern Europe and North America. It may reach a foot in height, with a disproportionately large head and hard, bony feet. The gnome can be expelled from the garden by swinging it in circles until dizzy and then dropping it over the garden wall. Alternatively a Jarvey may be used, though many wizards nowadays find this method of gnome-control too brutal.”
- Fantastic Beasts and Where to Find Them
Also, Harry is bitten by a gnome and does not feel any burgeoning talent. After he is bitten he does not feel or act any differently.
“Harry learned quickly not to feel too sorry for the gnomes. He decided just to drop the first one he caught over the hedge, but the gnome, sensing weakness, sank its razor-sharp teeth into Harry’s finger and he had a hard job shaking it off until –
‘Wow, Harry – that must’ve been fifty feet …”
- Harry Potter and the Chamber of Secrets, Chapter 3 (The Burrow)
The creativity-enhancing properties of gnome saliva are likely one of Xenophilius Lovegood’s strange and incorrect beliefs, as he is known to believe many things that are provably incorrect. His other belief about gnomes is that they are wise.
“She lingered in that charming little garden to say hello to the gnomes, such a glorious infestation! How few wizards realise just how much we can learn from the wise little gnomes – or, to give them their correct name, the Gernumbli gardensi.”
- Harry Potter and the Deathly Hallows, Chapter 8 (The Wedding)
However, Fantastic Beasts and Where to Find Them states that gnomes were placed firmly in the “beast” category when the decision was based on which creatures were not intelligent enough to understand or help shape the laws of the magical community.
“Not until 1811 were definitions found that most of the magical community found acceptable. Grogan Stump, the newly appointed Minister for Magic, decreed that a “being” was “any creature that has sufficient intelligence to understand the laws of the magical community and to bear part of the responsibility in shaping those laws.”2 Troll representatives were questioned in the absence of goblins and judged not to understand anything that was being said to them; they were therefore classified as “beasts” despite their two-legged gait; merpeople were invited through translators to become “beings” for the first time; fairies, pixies, and gnomes, despite their humanoid appearance, were placed firmly in the “beast” category.”
- Fantastic Beasts and Where to Find Them
Also, they’re shown as not being intelligent because they come out of their holes every time to look when they hear the Weasleys de-gnome their garden, and haven’t learned yet how that isn’t a good idea.
“The air was soon thick with flying gnomes.
‘See, they’re not too bright,’ said George, seizing five or six gnomes at once. ‘The moment they know the de-gnoming’s going on they storm up to have a look. You’d think they’d have learned by now just to stay put.”
- Harry Potter and the Chamber of Secrets, Chapter 3 (The Burrow)
Therefore it is highly unlikely that gnome saliva is used by wizards for its effects, because theses effects likely do not exist and are not believed in by anyone other than the Lovegoods believe gnome saliva has any effect.
