PJ - your not alone when it comes to this - many of us have it happen.
Just this summer a volunteer came to me that "everyone is talking about such and such and this person is saying this and that person is saying that, etc, etc." which then created this back and forth about this, that, and everything else....gossip and miscommunication.
I figured if those volunteers with questions or concerns can get their questions answered correctly or their concerns validated it would stop the gossip. We have a weekly volunteer newsletter that comes out so each week so I created "Ask Kim" (I'm the CEO and Co-Founder) where any/every volunteer can ask me directly about anything they want information on. This gives them (and everyone else) the opportunity to clearly understand what is going on and why but more importantly it takes away the opening for gossip conversations to take place. From me, they are getting direct and correct information, leaving no room for gossip.
If you don't have a volunteer weekly newsletter I would just look for something that gives your volunteers an opportunity to 1. be heard and 2. have what they have to say shared with others. That's key, sharing the information so for that reason I keep their questions anonymous.
The best part - out of 140 volunteers only 1 ever asked a question (but she asked 7!) so now I use "Ask Kim" to share information about the rescue with the all the volunteers on each segment of the rescue. This too worked great because they got to learn what each group does and why.
To avoid gossip just find a way to open up communication in a professional (and adult) way leaving those who gossip out on a ledge or out of your organization.
I hope this is helpful - hang in there.
#PeopleManagement