When I was a child, my parents complained (not in her hearing) about my grandmother's tendency to talk on and on about all the illnesses and deaths and general tragedies in the community. I agreed then and still do, because frankly, it's not something I want to dwell on-- especially if it's not someone I know well or can do anything to help.
But now they're about the age she was then, and lo and behold, they've started spending more and more time talking about those same medical problems and tragedies. They're not quite as bad as my grandmother, yet, but it's getting there. I sometimes wonder how much longer I have before I start doing it, too, and whether or not I'll notice when I do!
As for in-laws...
While they are very nice people, I don't expect to find my in-laws' conversation particularly interesting, most of the time. When we visit and are sitting around the table, I spend most of that time just sitting quietly... As long as they don't expect me to engage in conversation about these strangers they're discussing, it's not that hard to just let my mind wander.
However, if these visits are very frequent, I can see how that might become a strain. In that case, if you can't steer the conversation to something more universally interesting, I'd just try to limit the time spend sitting there. Maybe your partner can politely excuse himself after eating and take a walk or something while your parents chat with you.