In inline GPL you can set the distance between tick marks by the "delta()" option for a GUIDE statement. The example below would make the Y axis tick marks be 5 units apart:
GUIDE: axis(dim(2), label("Y"), delta(5))
From your description though it sounds as if you might be accidentally treating the Y axis as a categorical variable instead of numeric though. If making the plot through the graph builder GUI, make sure the variable level is set to scale when building the plot.