@Menglor - Stephen, looking at the visualizer picture down at the bottom middle of the screen. It is showing the dimensions that you specified. It’s showing X at 816 mm and Y at 1200 mm. Shouldn’t that be reversed? I think Length is Y and Width is X. I don’t know if that will correct what you’re showing or not but start there. Keep us posted please.
@Menglor - Stephen, stop the presses! I see the problem. You do need to reverse the dimensions first. Then look at the Start Position. It’s set for upper left back corner. Move it to lower left front corner.
In the surfacing window. Length should be 816 and Width should be 1200. Then Just below the Spindle M3 or M4 and just above the Generate G-code button is a box where you select clearing pattern. Either spiral or back and forth. There are four circles at each corner of that box representing the four corners of your cnc. Click on the lower left circle and then generate your g-code.
@Menglor - Also on the Start /Stop issue whenever you hit the Stop button gSender does a soft reset. That will most likely be the cause of losing your Z. Use the Pause and Resume buttons and that should keep all of your axes.