In simplest terms, if two players are playing a game against each other with game theory optimal (GTO) strategies, then neither player can gain an advantage by changing his strategy. This state is called a Nash Equilibrium. These are relatively easy to determine mathematically in simulations, depending on how complex the game is.

When you see "GTO" used here, it will usually mean a single player unilaterally using a strategy near the equilibrium strategy in order to avoid being exploited, and making money off of the parts of the opponent's strategy which are self-exploiting.

In simplest terms, it's non-exploitable strategy.

GTO is just a strategy (usually a really mixed one, where you want to make opponent hands to be breakeven between actions!). The nature of the GTO strategy is that its the maximum exploitable strategy of itself, so by definition if someone plays a GTO strategy and the other guy alters from it then he is losing EV because he is not playing the maximum exploitable (maximum EV) strategy.
Generally people kind of don't really understand math definitions, that's why this is so confusing. Hope this clears it up!

Is GTO relevant for small stakes? (Micros up to 200nl) or can i get away by mainly.focusing on expolitative strategy at these stakes

NOPE and yes i say : ) You should focus on exploitative strategy always, GTO is a framework + set of tools that you can use to sharpen those skills.