Gurgling, to me at least, means a slug of water wants to push past a slug of air. That's why I'd lean towards a venting issue.
Could be something simple like leaves in the vent, dead squirrel, etc... or not. Who knows? Anyhow, I'd investigate that first.
We started getting gurgling drains in our house. Come to find out the last owners installed a vent pipe from basement through the roof, but didn't secure it well, so it slid down about 3". That caused water to pool at the bottom of the vent pipe elbow, as it was the lowest point after it slipped. Every time we'd drain a sink or tub, eventually it would pull a slug of air through that pool and gurgle. I pushed the vent pipe back up from the basement and strapped it good. No more gurgles.
Good luck finding the culprit. With pipes, a sound you hear somewhere can be coming from across the house.