Guidance is one place where it really does not pay to buy a cheap GPS receiver.  First you need at least a 5 Hz update rate (which I think the G18 has), but you also need a stable smooth receiver.  Most of the "less expensive" 5 Hz GPS receivers tend to jump around quite a bit.  Then you end up chasing the line.  In the long run you will be more happy if you buy a quality receiver for guidance purposes.