Hey, that's a really great site for grabbing heightmaps!
Oh I see, it's drifting off to the side indefinitely. It could be that it's somehow not switching to absolute coordinates, so each absolute X coordinate just sends it off further and further down the X axis, in spite of the G90 that the Mach3 post includes at the beginning of the G-code. Maybe the controller is expecting it with some other G/M codes somehow. It seems like it could just be something finicky with the controller like that. Whatever it is I'd really like to know what it is and work in a fix - understanding the what/why of it.
It could very well be something else too, like the G40. There's no telling what the issue is without some trial-and-error just messing around with it. It could even have something to do with the lack of a "start read" character, the % line. I'd suggest trying removing that first line from the working G-code and see if it still works, or if its presence is inconsequential.
PixelCNC's posts are relatively simple, I tried to keep all the cruft out and avoid over-complicating anything with features and vernacular nobody ever uses 99.99% of the time. The most complex part are the codeblock definitions at the bottom where it references different variable names/states within the block's string of characters. You could really just directly replace PixelCNC's program prefix codeblock with the one that works for you, but knowing why the Mach3 post isn't working with your 6090 controller would be much more useful to others in the future. That way I can work in a fix without breaking Mach3 support for other controllers that the posts already work for.
Thanks!