Need Help newbie-Gerber 750 plus is not pen plotting correctly

From the looks of it, I'm going to take a wild guess and say there appears to be backlash in the Y-axis (the belt that drives the cutter head across the guide rails).

JB

 

I also would say offset for knife is still applied - looking at corners.

 

Might also look and see if its in Tangential mode and switch it to swivel mode.

 

I would clean off the chrome bars that the head slides back and forth on. Clean with alcohol and then put a small drop of 3 in 1 oil on the bars. These are brass bushings, so keeping the bars clean and lightly lubed is important. If this is an old plotter, they could be worn and would also contribute to this kind of poor plotting.
In the software, make sure that you're sending as a pen plot if you're using Omega to send the jobs. Tangential mode won't affect pen plots other than to slow it down and swivel mode will put hooks on the corners.

Good Luck