GPS vs Knotmeter
While a GPS will give you an average speed reading, it is over the bottom speed and not through the water speed. This is most significant when sailing in an area with a swift measurable current, such as a river, an area subject to tidal currents, or the ocean currents. It is totally unacceptable as a method to trim for speed, as you suggested, as it cannot react quickly enough and may be off-set by currents.
Same can be said of substituting a GPS for a helms compass. Be aware that you may be reading "True" courses, rather than magnetic courses. And also be aware that is is showing you your "Reading" or course to a particular nav point, not the direction you are pointing!
GPS are wonderful devices, but you have to use them for what they were intended for, a navigational aid. And then as a supplement to other forms of navigation. A person who relies exclusively on his "GPS/Map-color Model 4554" for navigation will have a rude awakening the first time the power is down on the batteries.
Can a GPS be used as a temporary fill-in while you save for your knot meter? Of course it can, but remember it''s limitation on showing it''s specific type of data.