I have something I need to get off my chest. It's a piece of information about myself that is particularly difficult to share on this board. I am deeply ashamed of this part of myself.
I like Gnomes. They're actually one of my favorite player races. They're in the top 3; just behind Warforged, just above Changelings.
Maybe it's that I haven't played any of the previous editions and was mostly introduced to them through Mordenkainen's Tome of Foes, or maybe it's just that some of their quirks are particularly relatable to me (having both ADHD and ASD). Or maybe my liking them is just objective badwrongfun and I should hide this part of my D&D-identity even more than I already do.
As others have said, Gnomes (like most other D&D races, if you're being honest) don't really have a single unifying theme, and haven't had one in decades. They're more of a mish-mash of ideas taken from folklore versions of Elves and Dwarves and similar creatures (brownies, hobs, nisse/tomte, some types of Trolls, etc), as well as a few modern additions to make them more unique (Tinker Gnomes from Dragonlance and World of Warcraft). They're Santa's Elves, Snow White's Dwarves, and mad scientists/inventors that constantly invent new types of explosives entirely by accident.
In D&D, races almost never have "unifying themes". They have grab-bags of ideas taken from pop culture and (typically European) folklore assembled into confusing amalgamations with taped-on lore that changes every edition (and sometimes in the middle of one).
In 5e, most of their broad themes are "quirky short people that look more human than Goblins but less than Halflings and have obsessions with either animals, tinkering, or gems/elemental earth". This is different from what it was in every previous edition, and is probably different from how they will be in the next edition.