Depends on what kind of arc you want to place.
It could be tangential from the last line segment, it could have an endpoint and a radius, it could have a centre point and a radius, and there are other versions as well.
Have a look in the GDL reference guide and look for the section on 'Status codes' and then go to the section for 'Additional status codes'.
Best is to drawer the shape (polyline) you want with one point on the project origin.
Then select polyline and go to FILE menu > Libraries and Objects > Save Selection as Library Part > Object.
Place the object, select it and open the script.
Have a look at the script to see what it has automatically written.
This might help you to understand.
It will probably only use this 4000 status code - start point, center point and rotation angle.
One of the forum moderators.
Versions 6.5 to 25
Dell XPS- i7-6700 @ 3.4Ghz, 16GB ram, GeForce GTX 960 (2GB), Windows 10
Dell Precision 3510 - i7 6820HQ @ 2.70GHz, 16GB RAM, AMD FirePro W5130M, Windows 10