In GSPPlot, check the Layout settings (type L to enter layout) to see if you have the checkbox checked for Center on Vinyl. I always print that way and was told a long time ago that it made a difference.
I don't use a non-Gerber plotter but I can vouch for the fact that target placement has always been quirky in GSPPlot. For example, when I print a paneled job, the bombsight target I use will be off about a quarter inch if I print it in the default setting of centered. It will be dead on if I print it in the lower left.
So the best advice I can give you would be to try different settings for Center on Vinyl and whatever choices you have with OPOS targets.
I don't use a non-Gerber plotter but I can vouch for the fact that target placement has always been quirky in GSPPlot. For example, when I print a paneled job, the bombsight target I use will be off about a quarter inch if I print it in the default setting of centered. It will be dead on if I print it in the lower left.
So the best advice I can give you would be to try different settings for Center on Vinyl and whatever choices you have with OPOS targets.