The tub and WM are not vented, so that is probably why you are getting the gurgling. The best fix is to run a separate vent up to the attic and out the roof (or you could tie the vent in 6" above the kitchen sink if there aren't fixtures from a higher level draining down). Running a new vent can be difficult and may requid repair work in you upper level(s). However, sometime you can run it in a corner of a closet and minimize the repairs.
The other option is an AAV (if allowed in your area). These are a mechanical device that lets air into the line, but not out. These need to be placed where they can be accessed (since they will need to be replaed down the road) and they also need to be able to breathe.