From their Conflict Handling section:

Gravel is a metaphor for small annoying problems that, if left unfixed, will get worse over time. For example a piece of gravel in your shoe. Best strategy is to stop and remove it as soon as you notice it.

Moose head is a metaphor for a big ugly stinking thing sitting on the table. It brings the mood down, but it’s been around for so long that few people notice it any more, and those who do don’t dare to mention it or do anything about it.

The purpose of our conflict handling process is to catch Gravel before it becomes a Moose head!