I’m pretty sure it can use the G2/G3 commands. One problem with GRBL is the extension is .gcode and Estlcam only accepts .nc extension. I played around looking at the post processor files and I could easily change the extension it creates, that’s all I know about it though. I don’t know if the extension actually means anything. Here are the preset from Estlcam CAM:
GRBL does not do anything with tool changes. That’s on the UI (Estlcam). If Estlcam reads the M6 command and is able to process it, then you’ll need a PP that adds them. I have a GRBL post for tool changes I can share (again, later).