You must be running a very old version of grbl. From version 0.9 forward grbl fully supports G2 and G3 commands and will draw an approximation of an arc when asked to do so.
If you should have a very large arc you will see some delay while grbl segments the arc into very short line segments to approximate the arc. One command to grbl.
If you let your CAM program break the arc up into hundreds and sometimes thousands of line segments which the sender must process then it becomes a matter of where you want your machine to spend its time.
Either wait for the G2/G3 to draw the entire arc with one command, or wait for the sender to send a huge number of line segments. It is selectable by the user with the Post Processor configuration.
I don’t know of any Cartesian co-ordinate machine that produces a true arc. Do you?