It never serves you to be part of gossip in the office. The people spreading gossip are coming from a negative perspective, often critical about management, how the company is being led, and they are not people with which you want to align your career. While best to not engage at all, you're bound to find yourself part of a conversation that turns to gossip, so know your options.

You can simply leave. Excuse yourself without getting involved as to why.
You can stand up and speak out about the fact that you are not comfortable in this conversation and don't think it's right to talk about others in this manner. 
You can speak up to your direct supervisor or boss about the concerns you have around gossip circulating and express your support in shifting the situation before it becomes a culture.  

Once you start engaging in gossip you align yourself with the group who talks behind peoples' backs. If you're looking to advance your career, you want to support the organization, be seen in the best light and act as a team player. Position yourself with the positive culture that exists in the company.  Hone your  leadership skills and keep your vision focused on your end goals to find success.