Gusle are mostly found among mountainous people here in Balkans as far as i know. Both Serbian Vojvodians and Croatian Slavonians and Zagorians etc don't use such instruments and these people are most Slavic of our nations. Gusle were mostly used by Illyrians that were under Byzantine influence, and Vlachs (and Albanians among others) thus we have them today among Serbs and Croats, and thats why Russians have Gusli as well, due to Byzantyne influence.
sorry for off-topic but i have read on the topic of Gusli before so just to clarify that more or less. But in the end instrument is actually as Dalibor said even drawn from Middle-east where it originated.
Original version of Gusli: