It’s really very simple Easel will not do anything with G2 and/or G3 (if in Easel design mode then Easel will generate straight line segments to approximate the curve) except return an error if it is asked to send a G2 and/or G3 command.
If you want to use Easel as your sending program the CAM program has to break arcs up into straight line segments. Controlling this behavior is in the realm of Fusion 360. Most CAM programs have to be told not to use G2 or G3 commands. This is done by choosing the correct post processor when you ask Fusion 360 to generate a toolpath (G-code file).
If you use UGCS you don’t have to worry about it you can just send the file. If grbl receives a G2 and/or G3 it will handle it. If the CAM program does not generate G2 and/or G3 then the CAM program will send straight line segments and grbl will handle it.
I understand why some people want to use Easel as their G-code sender, but I don’t really recommend it.
If you are still confused, speak up, I’ll try again.