The header is very machine dependent. Ideally, it would be user configurable.
Big things that are needed for Marlin are because G0 isn’t treated as a full speed travel move. The easy fix is to make sure the user defines a travel speed for xy and for z. Then make sure that speed is applied to every move.
The second big issue is that Z moves need to be slower. But that is the same fix. Make sure each line has a speed defined.
There are some special gcodes Marlin doesn’t support. Like drilling holes.
I am not the expert in post processors though. I haven’t made on. I just Monday morning quarterback them 