Dwayne is actually right on both accounts… when grease pencil was rewritten, they took out the guides and, at least as of 5.1, they hadn’t put them back in yet (circular or otherwise). However, you can draw a circle using the circle tool. Use Shift to constrain it to a perfect circle. Or you can freehand draw a circle and connect the ends. I believe if the ends are connected, the tools will treat the ends as continuous, but if they’re not connected, the tools will (finally) treat them independently if you have connected only fall off enabled (meaning if you edit one end of a non-connected stroke, it will respect the fall off as long as the fall off doesn’t reach as far as the other end of the stroke). When I’m doing freehand, particularly if I’m doing the line art, I’ll use auto-smoothing so the line isn’t jaggy, but turn it way down to like 10 so it doesn’t lag as much.
