Error Message example:

ERROR COMMIT-LOG-ALLOCATOR StorageService.java:413 - Stopping gossiper

What does this ERROR message mean?

This means that Cassandra is in the process of stopping gossip. This will usually be accompanied by native transport stopping as well.

Why does this ERROR occur?

This error occurs generally when there are other issue's within the cluster causing the node to crash. In general you will want to look for nodes being overloaded, or nodes crashing for other reasons (out of memory). It will usually be accompanied by similar error's such as:

ERROR COMMIT-LOG-ALLOCATOR StorageService.java:408 - Stopping native transport

How do you fix this ERROR

There is no fix for the error. The fix will come from looking for the reason why that node crashed. Likely, it is due to other issue's such as the node being overloaded, or the node crashing due to out of memory. It can also occur for issue's such as running out of disk space. In general, you will want to look for the reason that the node went down.
You can refer to the open source code here: Cassandra StorageService.java