In your first photo you used some sort of metal edging to make that straight outside edge. (It looks very nice.) Use the same or similar to create a circle that you fill with the gravel.

First, remove all the furniture from your firepit area. Find the center of your firepit with a measuring tape. Hammer a short piece of rebar or a landscape spike into the center of the firepit. Measure a piece of twine at least 6' long (12' is the minimum size for a firepit circle according to a Houzz Article) plus 3 inches extra. Loosely tie the string to the rebar or spike and use another spike or pointed stick to mark the distance from the spike to the edge of the area all the way around. This is where you place the edging. A 12' wide circle will require just under 38' of edging. A 14' wide circle will require 44' of edging. (diameter x 3.1416 = circumference) Fill the circle with your gravel, and replace the chairs.

This takes care of muddy dirt around the firepit, but you still need a mud-free path to this area, so plan on making a walkway to it like the others you are building.