Hi,
I have problem that if cambam add G21 to code it will cause mach3 to multiply all numbers with 25.4. If I remove that line then all works well and units are millimeters in file and mach3 but if there is G21 row it will multiply all units ~25.4. So it thinks that I have something in inch mode and it will try scale them to millimeters? What I need change that it will understand that all units are in millimeters in first place?

Config >Select Native Units, and set to mm.

If it was in inches, you'll need to change  your steps/unit in motor tuning and re-adjust your velocity and acceleration.

No, point is that g-code is in millimeters allready. If I remove that G21 line then all works fine.

Before you load the code that causes the scaling, look at the upper right of the screen. Is there a G20 or G21 in the status line? If there's a G20, then you're in inch mode. If it says G21 before you load the code, it shouldn't be doing any scaling.